Queen vs King....
Is there really a need for a larger bed? How much room do you have in your bedroom?
We had a king-size waterbed for a long time, and it was huge! We now have a queen regular mattress, and it is better sized for our bedroom. We don't "run into" each other on the queen bed.... it is fine for our needs.
As far as what kind, that really depends on you. One of your problems is that you each may have different preferences on mattress softness/hardness.
Personally, I don't like the tempurpedic type..... they make me feel hot. Others may rave about them.
For a temporary fix, check your box spring/foundation. Sometimes the boards break, and that causes sagging in the top mattress. For a temporary fix, you can put a piece of plywood between the two mattresses.
We have a double pillowtop... I can reverse the mattress and it has the pillowtop on the other side, also. We get firmness and softness with that.
For your next mattress purchase, be sure to turn the mattress as recommended... that will extend the life, and even out the wear.