Well, you just named all 3 roles and quite a few various classes
So it is hard to give some sane advice.
Personally, I like diversity, so being you I would've gone merc if I wanted to heal, cuz you already have op/scoundrel healer. If you want to tank, then sure, go PT. As for DPS, annihilation maras are nice for raiding. It is totally up to you, man. Just play what you want, lol.
I really like imp side btw for it's more sexy visuals and audio (in some cases) effects