Since you're running with shamans, I'd shy away from the pure melee. I'd go either a druid for PvP boomkin (that would be my choice) or a pally for a PvP healer. I don't think you're going to be playing a tank spec in PvP unless you're testing out a new combo.

Also, you can just run 4 toons in PvP. I like only running 4 - doesn't feel like I have too much responsibility and need to carry the whole team, while still having enough power to blow peeps up and survive an onslaught.