I’ll give you a bit of a breakdown on both.
Gallade (not mega) Stats- 68 HP, 125 Attack, 65 Defense, 65 Special Attack, 115 Special Defense, 80 Speed.
Gallade (mega) Stats- 68 HP, 165 Attack, 95 Defense, 65 Special Attack, 115 Special Defense, 110 Speed.
So, as you can see, Gallade is very physically oriented. With great Attack, good Special Defense, and decent Speed, it’s a pretty good Pokemon. To obtain Gallade, evolve a male Ralts into Kirlia. Then you can get a Dawn Stone at Victory Road (not recommended, as you will have a Kirlia for awhile), Sea Mauville, Super Training, and the Secret Pal in a Secret Base (source). Gallade’s STABs are super effective on Poochyena, Mightyena, Zubat, Golbat, and Koffing, as far as Team Magma’s Pokemon go. It’s STABs are also super effective against Roxanne (you probably won’t have a Gallade yet though), Brawly (probably still a Kirlia), Wattson’s Magneton and Magnemite, Norman, Sydney, Glacia’s 2 Glalies and Walrein, and Steven’s Aggron for gym leaders/the elite 4/the champion. Mega Gallade has really good stats, but unfortunately, you can’t obtain Galladite until after the Delta Episode (source, source 2).
Gardevoir (not mega) Stats- 68 HP, 65 Attack, 65 Defense, 125 Special Attack, 115 Special Defense, 80 Speed.
Gardevoir (mega) Stats- 68 HP, 85 Attack, 65 Defense, 165 Special Attack, 135 Special Defense, 100 Speed.
When Gallade and Gardevoir are in their base forms, their stats are the same, just Gardevoir having high SpAtk and Gallade having high Atk. Gardevoir also got the new fairy typing. All you need to obtain Gallade is to level Kirlia up to level 30, and the gender of your Kirlia doesn’t matter. For Team Magma, Gardevoir’s STABs are super effective against Poochyena, Mightyena, Koffing, and Golbat (please excuse me if I missed any other team magma Pokémon for either Gallade or Gardevoir). For the Gym Leaders/Elite 4/Champion, Gardevoir’s STABs are super effective against Brawly (most likely still a Kirlia), Sydney, Phoebe’s Sableye, and Drake. Gardevoirite can be obtained in Verdanturf Town after catching Groudon.
So, I suggest going with whichever one you’d like. They’re both pretty even.
Hope this helps! :)