Unfortunately this isn't a purchase you can rush - you have to try the mattress out. Next time you go to the bed shops, wear something comfortable and lie down for at least 10 minutes (preferably longer) so you can get an idea of support, pressure points, etc.
Also keep your base in mind, if you already have one. Some mattresses are meant for a specific type of base.
I've heard that the do-not-turn type of mattresses are cheaper because they have the good stuff on only one side.
Personally I'd buy a sponge mattress again. The Cloud Nine I have is very comfortable.