Why does it need to be either or

Do both.

Level another shaman on the pally account. (although I was going to suggest levelling him on one of your other accounts so you can pally boost the shaman from 35-58 using Zolo in Sunken temple. Then transfer him to the pally account when he has levelled.

I'm sure you'll find heroics where 5 shamans can't quite cut it (struggle more) so then you can bring the pally in for a bit of tanking.