Hi conway,
Welcome to the Mattress Forum! 
The first place I would start your research is the mattress shopping tutorial here which includes all the basic information you will need to make the best possible choice in any budget range … and perhaps more importantly know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for that are involved in each of them and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability, durability, and value.
Tempurpedic mattresses generally use good quality materials but they are certainly not in the best value range compared to many other memory foam mattresses that use similar or better quality materials that are available in much lower price ranges. There is also more about the pros and cons of memory foam in this article and while some people certainly like it … there are also many others that don’t so I would make sure that you do some testing on other types of mattresses as well to make sure that you really do prefer it over other types of mattresses and aren’t being overly influenced by the marketing that tends to surround memory foam mattresses because the choice of materials or which type of mattress you choose is a preference choice rather than a “better/worse” choice.
Every type of foam material has better quality and more durable versions and lower quality and less durable versions so no matter which type of material or type of mattress you tend to prefer I would make sure that the quality and durability of the materials are suitable and durable enough for your weight range.
Outside of PPP (Posture and alignment, Pressure relief, and Personal preferences) which is all about how well you will sleep on a mattress, the most important part of the “value” of a mattress purchase is the quality and durability of the materials inside it which is all about how long you will sleep well.
In terms of durability and the useful life of a mattress, a mattress is only as good as its construction and the quality/durability of the materials inside it regardless of the name of the manufacturer on the label and I would always make sure that you know the density of any memory foam and polyfoam in any mattress you are considering before making a purchase so you can make sure that there aren’t any lower quality or “unknown” materials or weak links in the mattress before you purchase it (see this article). Regardless of how it may feel in a showroom, the price of the mattress, or how well you may sleep on it initially … there is little value in purchasing a mattress that will only maintain the comfort and support which is the reason you purchased it for a few months or a few years before you need to replace it and the loss of comfort and support isn’t covered by mattress warranties (see post #174 here).
I would always keep in mind that there are no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that is firm for one can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type and weight, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. This is all relative and very subjective and is as much an art as a science. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here).
I would also be very cautious about using other people’s experiences or reviews on a mattress (either positive or negative) as a reliable source of information or guidance about how you will feel on the same mattress or how suitable or how durable a mattress may be for you and in many cases they can be more misleading than helpful because a mattress that would be a perfect choice for one person may be completely unsuitable for someone else to sleep on (see post #13 here).
When you are purchasing a mattress that you haven’t tried in person then the options you have after a purchase and the costs involved in returning or exchanging a mattress would become a much more important part of the “value” of a mattress purchase just in case the mattress you choose doesn’t turn out to be as suitable in terms of PPP as you hoped for. There is more about the Amazon large items return policy and the costs involved in post #11 here.
Having said all that … in the case of the Sleep Innovations mattress … the 2.5" of 3 lb memory foam would be a weak link in the mattress in terms of durability and I would avoid it or any mattress that doesn’t meet the quality/durability guidelines here. This would be even more important in your husband’s weight range.
You can read more about the Tuft & Needle mattress and some of the other “simplified choice” mattresses in the first two posts in this topic. Unlike the Sleep Innovations mattress it uses higher quality and more durable materials and there are no lower quality materials or “weak links” in the design in terms of durability. The high performance polyfoam they use in the comfort layers would have more resilience than memory foam which can help prevent the motion restriction or “stuck” feeling that is common with many memory foam mattresses as well. It would certainly make a much better quality/value choice than the Sleep Innovations mattress you are considering. They also have a free return policy so outside of the time you spend trying it or donating it (if it doesn’t turn out as well as you hoped) there is no risk in trying it because the only way to know whether any mattress will be a good “match” for your specific needs and preferences will be based on your own personal testing or actual sleeping experience.
Phoenix