Personally I recommend offsite. You get a much nicer hotel room for much less money. You can rent a car and park at the parks every day for much less than the difference in room prices and then you have complete freedom of movement. With our car we can get from any park to our hotel in 15 minutes or less. The idea of waiting for a bus to go anywhere and then cramming aboard is a real turnoff for me.
To be fair, I have no desire to be enveloped in the Disney "magic" 24 hours a day, though. DW and I enjoy the chance to unwind without the "magic". I also prefer the better and cheaper food options offsite, so even if we stayed onsite we'd probably leave for dinner frequently.
That said, if I found a comparable rate for a comparable room, I'd consider onsite.
To be fair, I have no desire to be enveloped in the Disney "magic" 24 hours a day, though. DW and I enjoy the chance to unwind without the "magic". I also prefer the better and cheaper food options offsite, so even if we stayed onsite we'd probably leave for dinner frequently.

That said, if I found a comparable rate for a comparable room, I'd consider onsite.
Eight days and counting down