I think the Fast Pass is more efficient this way. We chose three (hard to get the ones you want though) a certain number of days out from our trip (30?) and could still modify it while standing in the park. Once you've used your three, you can get another one. I think we only really did that once because the lines weren't horrible.
We wound up getting the park hopper at Universal. A huge amount of money! Ack. It was a great time though. Both parks have their strong points and both Harry Potter attractions were great.