We went through this several months ago, and we have quite a bit of trouble because DH and I don't necessarily match up on what feels good to us. We wound up getting ours at Mattress Firm. I don't know how long ago the other poster dealt with their return policy, but it was updated in the past year and it is very easy to switch them out now.
We initially got a Sealy. I absolutely loved it, but DH absolutely hated it, so we switched it out for a Sterns AND Foster. Mattress firm didn't give us any trouble at all about the switch. DH absolutely loved the Sterns, but I absolutely hated it. I tried for a few months on it, but the longer I tried, the less I slept at night and the more it made my back and shoulders (which I normally have no issues with at all) ache. We decided to move that one to the guest room and go back to Mattress Firm.
Sometime between the 2 purchases, Matress Firm installed this computer thing that you lay on a bed and it tells you which mattresses in the store are right for you. I thought it was completely silly when DH made me do it, and was very skeptical of it. Only one mattress matched up on both our lists, a Sealy. We bought it, and now we are both happy. So, if you have a Mattress Firm in your town I would suggest you try out that machine.
As for that Sterns and Foster I absolutely hated, my inlaws loved it so much in our guest room that they bought the same one, and now MIL says she is sleeping better than she has in years.