If this is in-game, Sylveon for better sp. atk, Aromatisse for better coverage. If this is competitive, Sylveon.
Sylveon doesn't learn a lot of coverage moves outside of its STAB, with the exception of Shadow Ball and Psyshock. Then again, they're all the moves that it needs, along with Calm Mind.
Aromatisse has abysmal speed, but at least it gets more coverage moves, like T-bolt, Energy Ball, Charge Beam, and Psychic.