trundle mattress

I'm looking to buy a trundle mattress for my son's twin bed/trundle set. I'm not real clear on what I should look for, I just want something basic, but comfortable enough for the occasional adult who might use it when there's a full house of guests. Since it will be used so infrequently, I also hope to keep it for a pretty long time and therefore need it to be pretty good quality (but of course don't want to spend a ton). Any thoughts on what you like/dislike about yours?

Any specifics would be helpful. I live in a fairly rural area, and will probably need to order online as opposed to going to a store to check them out in person. I think we have one small mattress store nearby, which I will check out this week.

Thanks!