Depends on the defintion of "bleaching," I think izoid and Amici are thinking it means the flesh falling off and seeing white skeleton, while I think it's the coral losing its pigment under too high of light.
Obviously the first defintion is worse, but if the second one is in play, I'd say browning--corals take forever to color back up from browning. Recovering from bleaching can be done in a month or so.