As far as I know pretty sure someone will correct me if I'm wrong :P
that nifty synergy of priest and warlock gonna be gone by next month.

1) sure they do about the same dps if they have about the same level of gear.

2) I agree they make decent healers.

3) shadow priest will be harder to maximize dps but yeh get stam buff , shield and fear which can be nice at times
whereas shammy yeh can drop some extra totems and do a lil aoe damage.
Its a toss up really