If you're talking about offences, Sylveon actually has better sp. atk and better speed. Sylveon's speed is at base 60, and Aromatisse at 29. At least Sylveon has a hope of outspeeding things.
The thing about Aromatisse is that none of its stats really stand out apart from its base 101 HP, whereas Sylveon has base 110 and 130 sp. atk & sp. def respectively. Aromatisse only has higher Attack & Defence and slightly higher HP than Sylveon, but attack doesn't really count because Aromatisse can't learn that many physical moves.
You also need to trade Spiritzee to evolve it, so that might be a hassle, whereas you can obtain Sylveon in-game pretty easily.
The only reason to pick Aromatisse over Sylveon is for coverage, as Sylveon (like all Eeveelutions) doesn't really learns any powerful moves outside its STAB, with the exception of Shadow Ball. Aromatisee can learn Thunderbolt, Energy Ball, Flash Cannon and Psychic, with the added option of Gyro Ball (since it's so slow).
As for Mawile, it just has the benefit of being dual Steel/ Fairy. It doesn't learn its best Fairy move until really late (L60), Play Rough at base 90. Mega Mawile is nice, but you don't get the stone until after beating the game. So Mawile is stuck with its base 85 atk. Normal Mawile has pretty poor stats, with none of them hitting green, and is only saved by its typing resistances.
Conclusion
Sylveon > Aromatisse if you want better sp. atk and speed.
Aromatisse > Sylveon if you want better coverage moves.
NB: also Fairy will fit well into your team, being another fighting resistance when you have 3 Pokemon weak to fighting, and also steel and poison weaknesses are covered by the rest of the team.
