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Well, I'm not actually a competitive battler, but I do breed competitive pokemon for the purpose of clearing out places like the Battle Maison (Subway, World Tournament, Battle Frontier, etc.) I generally look at the base stats of the pokemon I plan to work with to determine whether they might benefit from more speed or more hitting power. Now, usually, I prefer to have my pokemon with a lot of speed, but for pokemon that aren't terribly fast to begin with I try to balance it out with more hitting power and investing in some staying power.

When I look at Mega Gardevoir I see plenty of raw offense in that 165 base special attack so I think that 100 base speed could use more cushioning overall. On the other hand, I might enjoy playing a few mind games by giving it a Naughty nature to give it a little extra attack in exchange for some of that hefty special defense and give it Shadow Sneak for some priority. Even if it's on the weak side, sometimes that little dose of priority can make a huge difference. But that's me. I'm weird and I like do crazy experiments with move sets for my pokemon. But, getting back to something a little more standard, you can always raise Mega Gardevoir's special attack further with Calm Mind so I tend to think it would benefit more from additional speed since it has no way of raising it on its own.

And there I go drastically over-thinking things for a pokemon I don't actually plan on using. But yeah, I usually decide things based off the combination of stats and available movepool that I might want to take advantage of. Of course, sometimes I just raise three different versions of the same pokemon to test different sets.

I tend to be more flexible with my pokemon because I've trained up a lot of wild caught shiny pokemon (the result of an inordinate amount of grinding to get my favorite pokemon up to level 100 which in turn leads to even more grinding to train each new shiny thus becoming an unending cycle of new pokemon I want to train) that don't have the ideal nature or stats to run their more standard sets so I've had to think a lot about what will work best for them as individuals. It got me into the habit of improvising strategies with pretty much all of the pokemon I decide I want to train. Of course, that approach isn't for everyone, but it's something I've come to enjoy. But, again, I like to experiment and it gives me plenty of excuses to just have fun continuing to play the game well after the story has ended.

Anyway, I hope it all works out for you with your Gardevoir. Good luck!