Hi AED,
The first place to start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines you will need to make the best possible choice … and know how and why to avoid the worst ones.
With your higher weight it will be even more important to make sure you know the quality of the materials in any mattress you purchase because lower quality materials will soften or break down more quickly under higher weights. There is more information about buying a mattress for higher weight ranges in post #3 here.
Post #13 here also has more information about the most important parts of the “value” of a mattress purchase.
As you can see … the most important part of any mattress purchase is your confidence that a mattress will be a good match for you in terms of PPP (Posture and alignment, Pressure relief, ad Personal preferences) and that you will sleep well on it. This would either involve your own personal testing on a mattress or if you can’t test a mattress in person then a more detailed conversation with a knowledgeable and experienced online retailer that can help talk you through the options they have available that would have the best chance of being a good match for you and that is more interested in helping you choose the “right” mattress for your body type, sleeping positions, and preferences than they are in selling you anything you are willing to buy.
If you aren’t confident that you will sleep well on a mattress then the exchange and return policy and the costs involved would become a much more important part of your purchase decision so you can use your sleeping experience to decide whether your mattress is suitable for you and still have good options available to exchange or return it if it doesn’t work out as well as you hoped for. As far as I know overstock doesn’t allow returns on mattresses and they are certainly not knowledgeable about mattresses or in a position to give you good guidance on the phone so for me that would rule them out as a source for a mattress purchase in most cases.
There is more in post #2 here about the different ways to buy a mattress and how to minimize the risks involved with each of them.
While the thickness of the mattress itself is more of a side effect of the design than an important issue by itself … the type, quality, and thickness of the memory foam layer or layers will be one of the most important factors in whether a mattress is a good match for you in terms of PPP. A mattress that only has an inch or two of memory foam will tend to be quite firm and a mattress that uses cheaper lower quality/density memory foam will tend to soften and break down relatively quickly compared to higher quality/density memory foams. A quick scan through all the mattresses you listed seemed to indicate that the quality of the memory foam is lower than I would consider (see the foam quality guidelines here). Your budget range is probably too low for a good quality memory foam mattress that would be suitable for you and it may be worthwhile either raising your budget or considering other types of materials where you would be able to purchase a mattress that would use higher quality materials that would be more durable.
Some of the better lower budget online choices I’m aware of are listed in post #4 here.
There is more about the different ways that one mattress can match another one in post #9 here. I don’t know of another manufacturer that makes a mattress that they have designed to be an close approximation for the iComfort Savant (and it’s unlikely that there would be one in your budget range anyway) so the only way to know whether another mattress would feel similar to the Savant to you (regardless of the quality of the materials) would be based on either a more detailed conversation with an online retailer that was familiar with the Savant and how their mattresses compared to it or based on your own personal testing or sleeping experience. Outside of how it feels … you can compare the quality/density of the materials by finding out the quality/density of the materials in the Savant (which may be difficult to find out) and then comparing the quality/density of the materials inside the mattress you are comparing. Overall … I would tend to avoid using another mattress as your “target” and compare all the mattresses you are considering to a common set of criteria that are part of your personal value equation.
If you have a mattress that is still in good condition and there are no soft spots or visible impressions and all you need is some additional comfort and pressure relief then a topper can be a good idea. If you have a mattress that is past the end of its useful life and has soft spots or impressions then a topper isn’t the best idea because it will just “follow” the soft spots or impressions in the mattress underneath it. If you do decide to add a topper to your mattress then post #2 here and the topper guidelines it links to can help you decide on the type, thickness, and firmness that has the best chance of success.
Most foam mattresses (either memory foam, polyfoam, or latex foam) are flexible enough to use on an adjustable bed although once they are thicker than about 12" or so they may not bend as easily (depending to some degree on the specifics of the mattress) so you would need to check with the manufacturer or retailer to confirm they will work well on an adjustable bed.
Using a queen adjustable bed under a twin XL mattress may be a little awkward when you are getting in and out of bed but it wouldn’t harm the mattress. I would make sure that the retainer rails are positioned to keep the mattress from shifting (if they are in the corners it probably wouldn’t work). It would probably look a little odd as well if that’s important to you. If you are considering a queen size mattress at some point in the future the the tradeoffs may be worth it to you but these are the types of questions that only you can decide. There is more about choosing an adjustable bed in post #3 here and the adjustable bed thread it links to.
If I was in your shoes I would consider using the extra money to buy a better quality and more suitable mattress than the ones you are considering.
The better options and possibilities I’m aware of in the Tulsa, OK area that I’m aware of (subject to the guidelines I linked about suitability, durability, and value) are listed in post #2 here.
Phoenix