In my opinion if you are going to instance grind, lower is better for xp. For the most part you can clear all of blood furnace in 30 minutes at level 64/65ish and get 100K XP/hour without rested, This keeps up til you hit 68 and you can even push all the way to 69 there if you are a real masochist. About 1/3rd of the mobs gray out at 68 so the xp is slower then...

I personally balance between instance grinds and questing so I don't lose my mind. I find more than 3-4 runs through an instance in a row and I just want to slit my wrists.