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I've had a few futons over the years and I gotta say my favorite mattress so far (link below) was 8'' thick with springs. Some of the mattresses are thinner, without springs and for extended periods of use they can be uncomfortable.
I'd make sure the futon's sturdy. If you were to get a metal one (for example) make sure the support bars are thick! If they're not they'll start to bend before you know it.
Another suggestion is to make sure your futon has a metal stopper at the end of the frame. This keeps your mattress from sliding forward. If you have a thicker mattress or if you move around a lot you'll want something to keep your mattress from sliding forward which is where the stopper comes in handy.
This would be a good example of a metal frame with thicker bars and a stopper...
If you look at the front of the bed you'll see two bars that go vertical (I just call them stoppers, no idea what they're actually called.) Those are important to keep your mattress from sliding down!
And here's a link for a killer mattress (I'm using it right now for my futon which I sleep on every night.)