Hi Azelas,
What a great question … and one that hasn’t been asked in the whole existence of this forum! I certainly have some thoughts about this and while they may not agree with others (and some may legitimately argue with my opinions) … I’m happy to share them (and this will probably be a “reference” post for other similar questions).
The simple part is that like all the manufacturing members here … I think highly of the Select Foam mattresses and they certainly have great value compared to other mattresses using similar materials and construction (such as the Tempurpedic lineup). They also have the additional benefit that they have been constructed and “tested” in real life (using customer comparative feedback in side to side comparisons in their store as well as their own testing) to be a close approximation to the Tempurpedic lineup. While you are in the best position to decide which mattress is the best “model” possible (relative to your individual needs and preferences) … there is no doubt that their comparisons … unlike so many who compare themselves to the Tempurpedic line … are more legitimate and “close” in terms of layering and performance although this is also subjective and they use different types of foam so not everyone will agree that they “feel” the same. This also means that there is a widely available line of mattresses that can be tested locally to lower the risk of an online purchase. Just make sure if you do go in this direction you tell them you are a forum member here to get your additional 5% discount.
NOTE ADDED: Select foam used to be a member of this site but because of ongoing customer service issues and delays their membership was terminated and I would read the warning here before considering them.
But the more important (and interesting) part of your question is about "is a mattress over a certain somewhat arbitrary amount “worth it?”.
In essence my answer would be “it depends” and in the end it is something that only an individual can answer for themselves. But there is certainly much more to say … and I’m not one to be shy in talking about controversial subjects 
A mattress has two basic functions which is to provide pressure relief and to keep you in alignment in all your sleeping positions. Beyond these two basic needs … there are also many preferences which can be added to the equation such as microclimate and temperature control (perhaps the most important of the preferences and bordering on a “need”), motion isolation, subjective feel, response, and many others (which are the subject of post #46 here).
If someone is purchasing a mattress based on meeting their basic needs and preferences that can lead to deep, restful, healthy, and comfortable sleep, then there is no need for the large majority of people to go beyond this price range (for a queen size) or even less. There are hundreds of mattresses that can provide good pressure relief, good alignment, a good microclimate and temperature control, and for the most part the other important preferences as well for under this amount so spending more would certainly not be “necessary”. In other words … from a utilitarian point of view … and for those who treat mattresses as a necessity that is meant to support deep and healthy sleep rather than a luxury that includes other less necessary features … then the answer would be no.
But we are all individuals and our “value equations” may also include things that are in no way necessary or in some cases have little to do (at least directly) with the quality of sleep itself. For example … there are many people who will choose a car that is far more than they need (or in some cases can afford) because how they feel ABOUT the car is as important and part of how they feel IN the car. It may for example have the ability to reach 200 MPH or have faster acceleration that they never even use but they take great pleasure in owning anyway and makes driving the car a more enjoyable subjective experience. In other words … how they feel about their car is an important part of their personal value equation.
In the same way there are mattresses that cost in the multiple 5 figure range or more that are hand built by a team of skilled workmen (that have spent a lifetime mastering certain skills) that take several weeks to build a single mattress that use selected materials from different parts of the world in their construction and are as much a work of art as they are a mattress. While it is debatable if they provide higher levels of someones basic needs and preferences or would be more durable than a mattress that costs less than 10% of this amount … many individual subjective factors and how they feel ABOUT the mattress may be even more important to them than how the mattress performs and feels when they sleep on it. Some people would rather own a piece of furniture that was dovetailed by hand even though a machine may do a better job or have no discernible difference in it’s quality, performance, or longevity.
For some people who are fortunate enough to be in this position … the most expensive possible mattress may be part of why they choose to buy it because the highest possible price of a mattress (that they can justify) is actually part of their value equation. For some people … part of a mattress choice is it’s beauty and how it looks and fits their concepts as a “work of art” which may be just as important to them as the performance and feel of the mattress itself and make sleeping on the mattress a more enjoyable experience. Believe it or not … because of the effects of our subjective feelings about a mattress (or anything) … it can actually change our objective perceptions of the mattress as well and lead to a better sleep … even though the measurable performance may be the same as a much lower priced mattress.
In other cases … there are people whose needs and preferences fall inside such a very narrow range of tolerance that it excludes the more common types of mattresses, materials, and construction that most people would otherwise be happy buying. Examples of this may be someone who has a medical condition that requires a very specific type of mattress or material or a very specific type of construction that takes their narrower range and limitations into account. This would usually involve some type of custom building or special materials that for most wouldn’t be important. Someone with MCS (multiple chemical sensitivity) for example may be willing and happy to spend a great deal more on a mattress that used carefully selected materials that were more uncommon and expensive but that gave them the ability to sleep on completely natural materials that they didn’t react to. Others may be unusually sensitive or have such a narrow range of tolerance for pressure relief, alignment, temperature regulation, or other preferences that they may need a special type of construction or material that was more expensive but would give them the ability to sleep well and without pain or discomfort that others that are more within an “average” range of needs and preferences would never need, notice, or consider and would take for granted.
One other consideration that is important is that the quality, value, or expense of a mattress has nothing to do with it’s suitability for a particular person. A $40,000 mattress can be just as unsuitable and uncomfortable for a particular person as a $400 mattress … although it will likely last much longer :). In other words … a mattress needs to “match” each person’s needs regardless of it’s cost and then the “preferences” are a value bonus.
In the case of the specific mattresses you are looking at … both the SelectFoam Grandis and the Tempurpedic Grand Bed have very thick layers of memory foam which can be a risky construction with memory foam for anyone for several reasons. This is primarily because memory foam will soften with heat and humidity but also with time spent on the mattress because of a property of all viscoelastic materials called “creep”. This means that independent of the performance and feel of the mattress when it initially softens with your body heat and humidity levels … it will “relax” over time and you will gradually sink in more deeply with the heavier parts of your body. This can lead to situations where someone is in alignment in the first part of the night but wakes up out of alignment with a sore back in the morning. Whether this will happen to any individual person depends on the many individual variables of their body type and sleeping style, how frequently they change positions, how evenly they sink into a mattress, and how they interact with the mattress over time but the “risk” needs to be considered.
In addition to this … because memory foam is a less breathable, more insulating, and less temperature regulating material than other types of foam or natural fibers, in a mattress where you sink into the upper memory foam layers more deeply and because of the nature of memory foam to rise up around your body more closely than other types of foam (forming an insulating layer around more of your body surface) … it may also lead to heat and ventilation issues with regular overnight use than someone experiences in testing in a showroom.
So it would be debatable whether the Grandis or the GrandBed would provide you with any more of your needs and preferences than another lower priced mattress. It is also debatable how much of what you feel on either is because of how you may feel about the mattress vs what you feel on the mattress or whether there is some measurable or quantifiable difference between either and another mattress that would feel and perform just as well for you.
It really boils down to subjective factors that make one otherwise “equal” product (or in this case mattress) more attractive than another according to your own personal “value equation” and the effect that these subjective factors have on your overall long term satisfaction with the mattress. Is a mattress like this “necessary” or even objectively “better” than another lower priced mattress in a way that can be objectively measured … in most cases I would say no. Does it have various undefinable subjective properties that for any reason you feel better about buying and using and consider it more “valuable” than another less expensive choice … perhaps. Part of the answer to this depends on the degree that these subjective factors are affecting your objective perceptions or even just your overall satisfaction with owning the mattress.
In the end … as long as your needs and your important or “necessary” preferences are met and as long as you know any risks that are part of the mattress’ construction and have taken them into account … then the odds are good you will be happy with your purchase.
Is it “better” or “necessary” in objective terms than another lower priced mattress that uses less memory foam or has a less risky construction that would also meet your needs and preferences … probably not.
Is it better (or even necessary) for YOU for any of the reasons I have mentioned and is it “worth” the extra money? … only you can answer.
Thanks for asking such a great question
Phoenix