Either would be good to boost through lower level instances.

Round about Scarlet monastery I would probably lean towards the shaman. Being able to pop heals when needed would be quite good, and you should take less damage.

To solve the tagging issue you could put a spike on your shield. The mobs get tagged when hitting the shield.

I'd start out elemental and I would try bigger and bigger pulls before trying to pull the whole instance, just to judge how much damage the shaman will take.

I might even consider going Resto for healing pushback reduction and natures swiftness. (I'm borrowing a lot from the Zolo method that I used as a level 70 shaman in sunken temple before it was fixed - just search for zolo on the forum).

Just gather up as much as you can handle, make sure you are in range of your lowbies, put your back to a wall and burn the mobs down.

After SM I would head to Stratholme and stick with the shaman. Limit the size of your pulls and clear off diseases. This would see you to outland levels.