I'm mostly going to play 4e because it's supposed to lessen the complexity of the game, especially at high level. And because my players feel too entitled to all the gewgaws and widgets of all the WotC books, and I want to simplify things a bit with the start of a new campaign.
I fully expect to break down and start tinkering before we even play our first game, though, so who knows whether I might decide to take a crack at the magic system. Or I might get fed up with the gamist aspects of it all, and switch over to True 20.