I'm leveling through Outlands now. I did the starter quests in HFP, but then exclusvely worked through Ramps and BF (just cleared BF last night for the first time). I would probably get more XP doing quests, but I really like working my way through the instances. It helps me refine macros, and techniques, and helps me get the hang of the finer points of 5-boxing. I will most likely be doing a mix of quests and instances from here on out. I really like the Nagrand quests and the coilfang instances.