Hi TandL,
Gel memory foam tends to be a little bit more “supportive” than the same density of “regular” memory foam and can also make some difference in temperature regulation as well (although it may be temporary when you first go to sleep rather than over the course of the night and other temperature regulating factors will play a significant role as well).
At your weight you are “just in the range” that I would consider reducing the amount of 4 lb memory foam for durability reasons but having said that it would depend on the specifics of the mattress and all the other durability factors involved and if it comes down to a choice between PPP and durability I would choose PPP because I would much rather have a higher quality of sleep for a little shorter period of time than a lower quality of sleep for a longer period of time.
While there is no way to use a formula to make a choice without personal experience and it will ultimately depend on your actual sleeping experience … I would lean towards your “best judgement” choice based on conversations between you and the manufacturer.
When the layers are exchangeable … you will still have good options remaining after you have had the chance to sleep on the mattress and you will have a much better reference point both for the mattress you order and for any changes you may choose to make.
Phoenix