Haha I realized I just ranted about back problems and didn't actually answer the question
Size, you want at least a queen, king if you can get it. More room to roll around on, change positions, fit extra women into if you're a luckier SOB than I am. You want a solid frame that can hold up to lots of shaking and bouncing and movements. Water beds are neat a first as a novelty but get annoying after the first few times, its hard to get any kind of traction on them. They're hell on my back too so I stay away from sleeping on them. A firm mattress has more "bounce" to it. Hope that helps.