Outland and Northrend will give slightly more xp but Northrend can be a bugger as the bot tries to clip across the sea and dies from fatigue.
I typically let my toons go to outland after they ding 80 as it rarely gets stuck there and outland is small enough to make hitting dig sites quick.
Main thing is to get to 280% speed asap as that directly effects your xp/hour.