Hi jodiesattva.
Welcome to our forum. 
It looks like you are leaning towards the “better” of the two options that you have available, but for the wrong reasons. While Tempurpedic isn’t a brand that I recommend to members of the forum due to their overpriced nature, and as you need to choose between the two lines Tempur offers for your exchange, I suggest that you base your choice on your needs and preferences rather than the durability of the two different lines. Generally speaking, there is no point in having a mattress that will last you longer if you cannot sleep well on it. As you sleep prone, I would also avoid choosing something that has too much soft material in the uppermost layers. This being said, the Contour Supreme uses the following layers and components …
Comfort Layers
1" TEMPUR Comfort layer 5.3LB
2.4" TEMPUR Support layer 5.3LB
Support System
3" Polyfoam Base 2.0LB
5.1" Polyfoam Base 2.0LB
These are all high-quality materials and there are no lower quality materials or weak links in this mattress that would compromise the durability of the mattress, and being firmer would probably the best for someone seeping prone. You can read some of the past comments on the Contour Supreme here
The Flex Prima is in a different category from most of the other Tempurpedic mattresses because it’s a pocket coil/memory foam/polyfoam hybrid which uses memory foam and polyfoam in the comfort and transition layers on top of a pocket coil support core. This would have a different and more responsive and resilient “feel” compared to most of their mattresses that use a polyfoam support core with memory foam comfort and transition layers.
Tempurpedic doesn’t disclose the quality/density of the memory foam and polyfoam comfort and transition layers in this mattress but there are several inches of lower density polyfoam under the memory foam (probably 1.6 lb density) which would be a lower density and less durable material than I would normally like to see in a one sided mattress (see the quality/durability guidelines here) and I would normally avoid any mattress that has more than “about an inch or so” of lower quality or “unknown” materials in the upper layers of the mattress because it could reduce and/or compromise the durability and useful life of the mattress.
There is more information about the 3 most important parts of the “value” of a mattress purchase (or in your case a warranty replacement) in post #13 here which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on suitability, durability, and all the other parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase to fine tune the mattress or any exchange/return options that are available to you).
The first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort” or PPP or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
While I can’t speak to how any mattress will “feel” for someone else in terms of firmness, “comfort”, or PPP because this is too subjective and relative to different body types, sleeping positions, and individual preferences, sensitivities, and circumstances … outside of PPP (which is all about how well you will sleep on a mattress) the most important part of the value of a mattress purchase is it’s durability and in terms of durability a mattress is only as good as its construction and the type, quality, and durability of the materials inside it (which is all about how long you will sleep well) regardless of the name of the manufacturer or the name of the mattress on the label.
Hope this helps
Phoenix