There's no question: the PZ T-28. It is absolutely the best 4ch trainer there is. Yes, it's low wing, but it flies very well, has great characteristics and parts are very easy to come by. There is a reason why it seems like every electric pilot owns one at one point or another!
The wings do come off with a single screw, so dorm-room storage shouldn't be that big a problem.
The UM version is very fun, very durable, but is harder to fly and does not handle wind all that well due to the undercambered wing. I wish they'd make an AS3X version of it sometime....
The T-28 is the best second plane you can get, hands down. And it's the sort of plane that you will love to fly ever once you're totally comfortable with other 4ch planes. There's nothing I love more than shooting touch and goes with my T-28 time and time again...