for us, we learnt the following
1) the softer the mattress, the more expensive it is.
2) pocketed springs need a minimum weight before it will compress. so for super featherweight people like geri, a pocketed spring only mattress is almost like sleeping on the floor for her. =P
similarly, i can feel the difference between 4" of memory foam vs 2", but geri can't cos her weight not heavy enough.
3) the memory foam, whatever layer on top of the mattress is supposed to wrap around your body so provide the comfort, while the springs provide the stability. The more the foam wraps around your body, the better support you will get, and the pressure better distributed. Its like if you sleep on the floor, the bulk of the weight is on the butt, shoulders. etc. so the soft layers is supposed to help with that.
4) the $14,000 dunoplillo mattress that we tried feels the same as the $1600 dunnowhatbrand one. so just go and try any regardless of brand. anyway all got 10+ years warranty.
5) side sleeper and back sleeper will have different requirements. be sure to try both positions when testing.
6) at the end of the day, the main thing that i look out for, is how well it supports my lower back. must be able to feel support across the entire back.