I present to you the big list of comparisons between Sylveon and M-Gardevoir!
Sylveon:
Has more HP for wish-passing, and is bulkier overall.
Doesn't take up a mega-slot.
Gets access to an actually useful item.
Resists Bug, and is normally affected by ghost.
M-Gardevoir
Faster, meaning it can outpace things Sylveon can't.
More powerful, so if you were to run a CM set it would do better.
Psychic STAB, so Psyshock can do more damage to special walls.
Can burn stuff with Will-o-Wisp if you do want to run a defensive set.
I think this sums it up. I probably missed stuff, but this should cover all of the relevant concerns.
Sylveon:
Has more HP for wish-passing, and is bulkier overall.
Doesn't take up a mega-slot.
Gets access to an actually useful item.
Resists Bug, and is normally affected by ghost.
M-Gardevoir
Faster, meaning it can outpace things Sylveon can't.
More powerful, so if you were to run a CM set it would do better.
Psychic STAB, so Psyshock can do more damage to special walls.
Can burn stuff with Will-o-Wisp if you do want to run a defensive set.
I think this sums it up. I probably missed stuff, but this should cover all of the relevant concerns.