My biggest beef with Guardian as FC is that the Guardian armor and stam bonuses just aren't as nice as the Resto heals. I just live longer as Resto. And using your Guardian stun when holding the flag is just silly so you kinda loose access to that perk as a FC. Here's what I run:
Guardian if I want to play D (seems to be the only spec where I can slow down and sometimes even kill fellow Druids 1v1).
Feral if I want to play O.
Resto if I want to FC.