It is actually possible to have both setups on the one car. I have seen a video where it was done on a 2000 WRX in whereby standard he was driving AWD, and then when he wanted to switch to 2WD, he would pull a fuse out from in the bonet smewhere and the car ran on two... I dont know how he did it or what electronics and drivetrain mods where involved, but it was done. And in the video it was kept very secret. From memory I think it was from a Fast fours or Hot 4's video.... going back years now, so I cant remember the full story.