I wouldn't be so sure about that... If instances are as difficult as people have claimed they are your group could be wiping more than you think, which might drastically reduce your XP gain per hour.
But I guess fighting for quest mobs in Hyjal/Vashj'ir is going to be just as difficult. Ah well.
As for me, my option isn't in the poll since I'm going to take it easy, leveling as I see fit but not rushing. Plenty of time to hit 85 and gear up before the raids get old and stale.