Mag had proven upside in that he'd previously been one of the best defenders in all of college basketball, but was coming off injuries. Wahlin has never really been good, but he has some physical traits that could translate if he can bulk up and polish his game. He's also a junior, so we don't have forever to stash and develop him. So it is a bit of an apples and oranges comparison, but I think Mag comes out on top.

EDIT: You said offensive upside, oops. I probably agree with that, but Mag is shooting almost 39% from 3. Tough call.