Which one should i choose out of these two?

My team is

Greninja
Mega Absol
Hydreigon
Meowstic
Charizard

If i go with Gardevoir I will have a fast Special Attacker to take out dragons but if I got with Mawile it will be effective against both Fairies and Dragons with Steel/Fairy.

Any recomendations would be nice.

Gardevoir, mawile got boost but Gardevoir is greater overall (mawile better typing though)

Gardevoir is great with double battles in a competitive battle imo (not counting mega's)

Personally Id go with Aggron. You need at least one pokemon with high defense and mega aggrons defense can go over 600

Mega Aggron is a beast, best physical defense pokemon by far. Great atk and being pure steel with defenses that tie Shuckle but with way more health and to note its weaknesses (fire/ground/fighting) do 25% less damage! A freaking beast

I'd go with Gardevoir, but like ElToucanSam stated Aggron would be a great addition. Is this the team you are set on? I'd swap out Meowstic (this is a fairy type right?) for Aggron. I'm guessing you are playing X, so Aggron would fit, but I read Aggron's mega stone is only available in Y (not confirmed), and vice versa for Tyranitar.

Off question: When did you get Deino? Was it early into the game?

No im not set on this team, but i like to play with new pokemon first playthrough.  Meowstic actually surprised me that it was overall pretty decent all across the board.  

And i just got him today.  Tbone traded me one :D

Yeah, I was going to do that, but I haven't run into that many amazing must haves yet (just beat 4th gym leader). My team is Delphox (obviously new), Venusaur, Lapras, and Gabite. Plus I need to HM slaves for fly, cut, strength and occasionaly, rock smash. I was thinking of adding Slyveon. Have you, or anyone else here use it? I also do not want to look online at a full pokedex and plan my team.

meowstic has different gender appearance as well as a different movepool! They both have a base stat of 104 speed which is great and a good sp atk!

My friend just started using Sly and he says it is amazing.

Im using male, love it's design.  But what surprises me is how good it's defense is.  Its only flaw s attack and that is useless for him anyways