Personally I would use the shammy as the main, as I find it easier to play the char who can take more hits and has the better aggro pulling ability. This ceases to be the case when you have enough dps to drop things before they reach you, which probably isn't the case with the classes you are picking at least until you hit the higher levels. I dual box two mages as well as two shammies, and the mages can keep things from closing really easily, while the shammies aren't quite as effective and end up in melee range most of the time after the initial pulls.