PVP wise for once all the specs (except guardian ofc) seem to be very competitive. Ferals can break their roots and slows again and still deal a lot of damage, balance comps in cata will work even better in mop and resto is probably going to be the best healer for arenas in mop.
Pve wise balance seems a little disappointing. Resto is performing well but is not the best. Feral looks pretty good. Guardian is lately getting some nice buffs and will be a pretty good tank I guess.
I will be playing resto for both pve and pvp, and balance for only pvp but they are all viable at everything so its mostly about what you enjoy the most.