Consider where you'll put it, before you buy one. I bought the ultra cheap one sold by Trident. It's 10 feet tall, but made of such cheap plastic (like a 10 foot tall red plastic bag) that it rolls up very small. All but one of the times I've deployed it, someone has comented how much better theirs was ...but they couldn't show me how great theirs was, because it was at home. If you don't bring your sausage for drift dives in the Carribean, there's no point in owning one.
I recently added a "handle" (made of duct tape) to my tube. (As sold, you just crumple the bottom of the tube, and hold the open end under water.) The captain of the boat said, "That really worked well. Did you buy that like that?" I laughed, and said that I'd added the duct tape on my own. Personally I prefer the added height of my 10' tube, in case of waves that might hide a 36" tube.
Whatever you buy, practice deploying it once in a while (when you don't need it). You don't want to have to figure out how your going to inflate it, while you're already have a bad dive.