I agree with rotordave (who, apparently, has done everything). As stupid as you feel laying on a bed while a salesperson stares at you, it's the best way to find out what works best. My wife and I recently replaced our antique (full size) bedroom set with something new (yet primitive for HER tastes). We bought a Sterns and Foster pillow top firm mattress. Old company and a quality name. We didn't want to spend more than 2k on a king mattress and this was the best option that we were BOTH comfortable on.
Best advice...GET A GOOD MATTRESS COVER! Even if you don't have animals, it keeps the mattress looking clean for years.