Well, my elemental shamans are level 50 now.

I leveled from 48-49 through questing. It took me a good 3 hours in hinterlands. Which is a pretty quest friendly zone for horde multiboxers. (and another hour worth of traveling/slacking which I wont count).

Then I powerleveled my four alts in lbrs with my main (Enhancement shaman, so no AoE, epic gear and 1200 dps tho.) and it took me 90 minutes to get to 50, without the traveling/slacking time.

So, even without AoE, it seems instance grinding is about twice as fast then questing. Oo

The problem is, I can never keep grinding for more then like 2 hours myself before getting totally demoralized and just slack/do other stuff instead. But if you can bring it up to do some massive grinding, instances is probably the way to go. Boost them or have them get boosted. (I sometimes get a friend to boost my chars for me a run or two, it's quite laid back and since you're not alone also more fun!)