Both are terrific in-game mons, can't even lie. Normally I would say Gallade due to that fantastic learnpool with moves like Close Combat, Leaf Blade, Psycho Cut, Night Slash, and Swords Dance, but I gotta give it up to Gardevoir because of its hella good typing. With the Fairy Typing, now it can smack Dragons as well resist resist hits from Fighting even better than before as well take neutral damage from Bug and Dark types. That does come with the price of becoming weak to Steel and Poison but it can smack Poison-types with Psychic and switch out if facing a Steel Type/hit that sucka with a Focus Blast when we get the TM for it. However, compared to Gallade, Gardevoir learnpool is pretty lame with the only good moves are Psychic, Moonblast, Stored Power, and Calm Mind
Personally, I would go with Gardevoir, but it all depends on what your team comp is mate.