Your best bet is to head to the local outfitters and try them all out. Don't overlook the new cots that Thermarest brought out, since the boat will carry it. I haven't seen them myself, but they may be worth looking at.

Also, have you considered a hammock? I haven't ever used one, and I don't have back problems, so I don't know how they would work for you, but you might check them out, just to see if they worked better than a pad on the ground.

My own personal favorite is the Q Core from Big Agnes (I like the SL, but the regular Q Core is also comfortable, and may come in a wider size, if that's a factor.)

Good luck.