Currently neither one is that important, it all depends on what raid bosses we get after the event or further into the future. If you have some decently high level Golems, they will be better than the B/SE Tyranitars, so I would CTM those to have Crunch.
EDIT: B/SE Tyranitar is worse as a dark type attacker than B/C Tyranitar and worse as a rock attacker than double rock Golem. There is no reason not to change it provided you can spare the CTMs.