Also, Gallade's and Gardevoir's movepools are very different (not counting TMs).
Gardevoir can be a wonderful support (Wish, Trick Room, Hypnosis, Healing Wish)
or a decent Sp. Attacker (Psychic, Moonblast/Draining Kiss, Dream Eater, Calm Mind...)
While Gallade is a speedy Physical Attacker (Psycho Cut, Leaf Blade, Night Slash, Swords Dance, Close Combat)
In my opinion, Gardevoir without TMs hasn't a good offensive power, she has only Psychic and Fairy type attacks.
Gallade has a great movepool, but you need to go to the Move Tutor/Reminder to teach him some moves like Night Slash.
Summing up:
If you don't have many good TMs and want an offensive power, choose Gallade.
If you have good TMs and want an offensive power, choose Gardevoir.