I asked a similar question about a month ago.
We went on Sep 26. Very quiet. We bought a one day, one park, pass on-line, just to see if we could even do 2 parks.
We arrived late 10:00 AM. We hit IOA first. We were able to do the major attractions in 2.5 hours. We did miss the stunt show. We then went to guest relations and bought the 2 park upgrade.
We then went over to US and did, ET, BJGR, earthquake, Jaws, nicklodeon ride and Shrek. We even included lunch at the burger joint with the vintage cars.
Theer were no lines. Only waits were when we arrived between showings (like Shrek). I would go and upgrade when you got there. Its a painless process, and really doesn't save much (any?) to pay in advance.
By 5:00 we had enough and went back to WDW (it was starting to pour, plus we had a lllloooongg walk back to the car).