It doesn't really matter in-game. Trace is a tiny bit better, as you can copy potentially helpful Abilities like Intimidate, Water Absorb, etc. However, Synchronize is also good, for the mere fact that you will be randomly statused quite a bit over the course of the game.
Gallade isn't very viable, as Dawn Stone comes way too late in the game, meaning you have to lug a Kirlia around until just before the Elite 4.
Hope I helped!