Play both proficiently, then pick the one that is demanded most with the event that is required for. No need to "jump ship" when it's possible to play / gear both up effectively as the only real gear that is not multi-class is the myth pieces and the weapons.

[EDIT] Yes, BiS will affect your final gear choices, but it shouldn't affect your performance that dramatically that it'll cause you fail at your role if you are a proficient player.