As weird as it sounds I think it's going to be similar to how Elemental was in Cata. They had Haste for Arena, and Mastery for BGs/5s/RBGs.
Haste for 3s,
Crit for Random bgs, Rated Bgs and 5s - possibly? I feel like Crit really shines when you get 4-5 + targets DoTed. Where as in 3s, getting off that cyclone or hard cast healing touch is far more important and likely to get kicked. In smaller groups my kills (At least the wargame 3s I have been running) is coming from hard casting.
Whats everyone else's opinion now that we have been 90 for a little while? =)