It doesn't really matter in game --- both are pretty niche and Gardevoir doesn't rely on its Ability. Trace is rarely relevant, though it can result in some funny situations like Levitate Gardevoir walling Drake's Flygon. Synchronize is similarly unnecessary and won't be winning you any otherwise-impossible fights.
Trace is slightly better overall because it's useful in fringe cases like Drake's Flygon and Stevens's Claydol. Neither Ability will be particularly helpful in the majority of battles, however.
Gallade isn't very viable, as the Dawn Stone is only obtainable in Victory Road. Using a Kirlia until the end of the game is neither practical nor fun and Gallade isn't strong enough against the Elite 4 to make up for it.
Hope I helped!
