Which Mega to use?

So I'm gonna be using Sceptile, Aggron, and Gallade, along with Crobat, Exploud, and Lanturn, all on my storyline team. I'm wondering, which mega should I use between the first 3? Sceptile has a quad weakness to Ice, but it looks rather badass and seems to be the best (but, it is a starter, after all). I don't see any big negatives or big positives between Aggron and Gallade, but I'm just not sure. Remember this is for ingame, not for competitive, so keep in mind of the E4 and late game gym leaders, etc. Thank you.

Edit: I realize now that Galladite is only obtainable after the delta episode, so now its just between Aggron and Sceptile.

I dont see why you couldn't just switch it up? carry both mega stones with you, and if you need to mega evolve to give yourself a type advantage during a gym battle or before each elite four member, then do it!

I was thinking of that. I don't really know how mega evolution works, I never used it in X/Y (none of the 'mons I decided to use had one) but I'll do that. Thank you.