First, let's look at their pros, cons, and recommended movesets:
Gardevoir
+125 base Sp. A
+Awesome sweeping capabilities, with Mega
+Access to the combination of Pixilate and Hyper Voice
+Great Mega Evolution
+versatile movepool, with access to a whole ton of coverage moves
-Its HP and Defense aren't the greatest, at 68 and 65, it won't be able to take many hits on its Physical side.
-Its 80 base speed may be ok to outspeed some threats, but doesn't cut it when faced with faster common pokes like Gengar.
Now for its set:
Gardevoir@Gardevoirite
252 Sp. A / 252 Spd / 4 Sp. D
Ability: Synchronize/Trace/Telepathy --> Pixilate
Timid Nature
-Hyper Voice/Moonblast
-Psyshock/Psychic
-Shadow Ball
-Focus Blast/Thunderbolt/Calm Mind/Energy Ball
Common sweeper set. The ability doesn't matter, because you'll be mega evolving it on its first turn out anyways. You may want Moonblast over Hyper Voice, because of the extra base power and Sp. A lower, but it's up to you. Psyshock, at times, may be more useful than Psychic, but it's up to you. Shadow Ball is Ghost coverage. The last slot can be used for a whole variety of things-more coverage, setup, etc. It's all up to you, and what your team needs most.
Gallade
+As a Mega, it gets a whole ton of upgrades to Abilities and stats.
+Inner Focus allows you to attack on a Fake Out user, like Mega Kangaskhan or Ambipom, and then possibly take them out.
+At 165 base Attack and 110 Speed, it has very good sweeping potential, possibly better than Gardevoir's.
+Very good movepool, giving it access to a lot of moves, such as X-Scissor, Earthquake, Ice Punch, Power-up Punch, Poison Jab, Leaf Blade, and more.
+Good Sp. D. Why does it need it anyways...?
+...It looks awesome...
-Without its Mega stone, it can't do much...and the things it can do are done better by other Pokemon, so if you want this on your team, you pretty much have to have it as your Mega.
Now for its set:
Gallade@Galladite
252 Atk / 252 Spd / 4 HP or Sp. D
Ability: Steadfast/Justified-->Inner Focus
Jolly Nature
-Close Combat/Drain Punch
-Zen Headbutt
-Ice Punch
-Night Slash/Knock Off/Power-up Punch/Leaf Blade/Fire Punch/Thunder Punch/Poison Jab
Again, common sweeper set. Close Combat offers more power than Drain Punch, but Drain Punch provides recovery that he could need.
Zen Headbutt is STAB and more powerful than Psycho Cut, at 85 compared to Psycho Cut's 70. Ice Punch is Flying coverage, but you could optionally use Thunder Punch over Ice Punch, just Ice Punch hits more things super-effectively. The last slot is just your selection of coverage, you could even run Earthquake if you wanted.
Conclusion
The Bottom Line
Gardevoir and Gallade both have amazing stats, good moves and sweeper potential. Gardevoir's typing is slightly better, while Gallade's Speed is superior to Gardevoir's. What I will say, is that whoever fits into your team better, use it. Try them both out! Do a few Showdown tests and see who you needed/is more important, then put it in.
