Our experience with a platform bed, traditional mattress, and a memory foam topper is that the memory foam is OK in the winter, but in the summer it's much softer so you sink down more and it's HOT. My husband thinks it's causing neck pain.
We're looking to change. Not an inexpensive option, but you might want to look into flobeds.com. You customize all the components, and they will switch them out if it doesn't work for you. Our neighbors love theirs, so we're considering switching before the weather turns warm again. I like the fact that they use natural fibers like wool.