Hi rgent,
The only way to know which mattress is the most supportive (or to be more accurate keeps you in the best alignment in all your sleeping positions) would be through your own personal experience and testing. Post #2 here has more about how to make the best possible support/alignment and comfort/pressure relief choices.
In terms of durability … a mattress is only as good as it’s construction and the quality of all the materials and components inside it. unfortunately you aren’t likely to be able to find out the quality/density of the foam layers in either of these mattresses so there is really no way to make any meaningful comparisons in terms of durability or the useful life of either mattress.
Having said that … and even though they are both a “blind purchase” (which would be like buying a piece of furniture that had a cover over it where you weren’t able to find out what the furniture was made of except that it was “some kind of wood product”) … between the two of these I would be leaning towards the Therapedic in terms of durability if for no other reason than it’s two sided which is a more durable construction than a one sided mattress. While Boscov’s isn’t likely to do this for you (which all retailers should be doing for their customers) … you may also be able to call the Therapedic factory yourself to find out the specifics of what is in their mattresses.
There are far too many variables and unknowns and individual preferences involved for anyone to recommend a specific mattress out of all the thousands that are available so the information on this site is designed to help you with how to choose rather than what to choose (which is unique to each individual). Post #1 here includes all the basic information, steps, and guidelines that are most helpful when you are mattress shopping but unfortunately you are somewhat “stuck” in choosing only between mattresses at a specific store that I would normally have suggested avoiding in the first place.
The comments in a recent post here would be similar to your situation but at at least you have the option of a two sided mattress which would likely be a better “blind” choice than most of the one sided mattresses made by the major bands … as long as you have done some careful and objective testing for PPP.
Some of the better possibilities there would be the Eclipse and Therapedic mattresses (and I would lean towards two sided in any blind purchase) and perhaps the Spring Air here which uses latex in some of the comfort layers which is a much more durable material than polyfoam and is a firmer version which would likely be more suitable for your higher weight. Again though they don’t provide the specifics of the rest of the layers which may be the weak link of the mattress.
Phoenix