The one thing I would look for is a return policy. You can't really tell if you'll like it from a two minute lay on the showroom floor. A friend just spent $1000 on one and he hates it but is stuck with it.
good luck
I would get a fairly firm, regular mattress that is at least queen sized. if it's too firm, you can soften it up with mattress toppers which is what I did. Do not let any salespeople talk you into any pillowtops which will quickly sink.
Try the king size for the two of you.
My boyfriend and I will be buying our very first mattress We don't know anything and don't want to be suckered into something by a sales person.
This is what I want in a mattress..
1. Good support but without it being very firm I like some softness
2. hypoallergenic I have allergies
3. no sinking !!!
4. relieve my boyfriends back pain
5. one that i can flip.
a few things u should know about how we sleep. my boyfriend sleeps on his back or side I mainly sleep on my stomach.