Hi mjgh06,
The quality/durability guidelines here suggest “no more than about an inch or so” of lower quality/density materials in the top layers of a mattress and once the total is 2" or more it could be a weak link in the mattress. In a one sided mattress lower quality/density means polyfoam that is less than about 1.8 lb density or memory foam that is less than about 4 lb density.
This means that either the top 1.5" layer of 1.5 lb polyfoam or the 1/2" layer of 3 lb memory foam would be OK by themselves but together they add up to 2" so this would be “on the edge” of being a weak link in the mattress and would be a more risky choice.
Again … I would keep in mind that mattress warranties only cover manufacturing defects in a mattress and they don’t cover foam softening and the loss of comfort and support that goes with it (which is considered to be “normal” and not a defect) and the loss of comfort and/or support is generally the main reason that you would need to buy a new mattress. Warranties have little to nothing to do with the durability or useful life of a mattress or how long it will be until you need to purchase a new one and longer warranties are more about marketing than anything else (see post #174 here).
Phoenix