I dont know wich char I will play as lvl 70 , the healer shaman or a dps one ?
The answer to this question is always the same: which one requires the most attention?

If I have a tank character (or, God forbid, I ever wanted a melee DPS character!) in the group then that one has to be the main, because positioning is so important.

If it's an all-caster group (which I think is where you're heading) then it makes no real difference, but then I favour having my healer as my main.