Whatever choice you make, it's worth a little extra money to buy an egg-crate foam piece to go on top. Our guests have commented how much nicer that makes our sleeper sofa. We just store it right behind the "head" of the sofa in our den.
Another thing to think about is firmness. Most sleeper sofa are very firm to sit on as a couch, so if you plan on using it as a couch a lot more than a sleeper, sit awhile on the one you plan to buy (and remember your new one will likely be stiffer than the store's demo model).
PHXscuba