Maybe you should focus on what each is weak to. Gardevoir is weak to these types: Poison, Ghost, Steel. Gallade is weak to these types: Flying, Ghost, Fairy. It also depends on what game you are playing: X/Y, Alpha Sapphire/Omega ruby, or Sun/Moon if you want to mega evolve it.
You can get Gardevoirite in X/Y, but you can't get Galladite.
You can get both mega stones in Omega Ruby/Alpha Sapphire.
You can't get any of the mega stones in Sun/Moon.
If you get enough BP at wherever you get BP in your game, you should be able to teach Gardevoir Hyper Voice. If you mega evolve it, it's ability becomes Pixilate, which turns any normal type moves it knows into fairy type moves. When a mega Gardevoir uses Hyper Voice, it does MASSIVE damage.
I don't know much about Gallade, but I've heard, that as a mega it is awesome. I've also heard that Gallade can learn a bunch of useful moves like Ice Punch to deal with any flying type Pokemon that are in your way.
So, in the end, it is really your choice.