Just do both.

I actually levelled 2 Shamans and 2 Mages, with a 3rd Shaman along the way.

Mostly I leveled 1 Mage + 2 Shamans, because having Blizzard and boositng with a Pally tanking AoE was great.

When I hit Outlands, I find the 3 Shamans more enjoyable. So the 2 Mages remain at 60 while I plow thru dungeons and quests with the 3 shamans (2 Ele, 1 Resto).

I do plan to level the 2 Mages still, but I think I'm going to enjoy BGs with the 3 shamans.