I've been playing WoW since Jan '05, but only started twinking in Oct '09. Therefore I've only ever twinked in XP off brackets. I have messed around a bit with my hunter who had enough xp room to Q xp on without dinging, and stomping people was fun to blow off steam, but it got boring fast. Then again, playing hunter in xp off gets boring fast for me...
My vote goes for XP off no doubt. It is far more rewarding to win a hard fought victory than it is to stomp noobs.