1: The Stat comparison
Special Attack: 85
Special Defense: 75
Special Attack: 99
Special Defense: 89
Slurpuff's HP stat is weaker than Aromatisse's, 82 < 101.
Slurpuff's Attack is better but not by far than Aromatisse's, 80 > 72.
Again, Slurpuff has the best defense out of the two, 86 > 75.
However, Aromatisse beats Slurpuff with the special attack stat, 99 > 85.
Aromatisse is also the best with the special defense stat, 89 > 75.
Slurpuff finally gets a win in the Speed factor, 72 > 29.
These two fairies are well evened, but as a total wise, Slurpuff wins it.
2: The relevant details
Slurpuff's stats are well evened. It gets access to Sweet Veil to support your team. Thus, none of your Pokémon can get asleep. This is half-decent. Hypnosis will no longer work on you, nor Spore. I find it pretty occasional, finally. Unburden is its second ability. Oh but come on! You should be using it with berries or gems. It will afterward double your speed, leaving you with a nice sweeper. Although it lacks a bit of special attack, Slurpuff has not a bad movepool. I mean Flamethrower, Energy Ball, Dazzling Gleam and Thunderbolt forms its attacking core. You should give it a Sitrus Berry with the four move above, to get something like this, if Slurpuff shall be your sweeper:
Slurpuff @ Sitrus Berry:
EVs: 252 Sp.Atk / 196 Speed / 64 HP
- Dazzling Gleam
- Energy Ball
And if you want a supporter, it gets Safeguard, which prevent status ailment hitting you, Aromatherapy, Cotton Spore, Light Screen and that's pretty it. Do not use Slurpuff, it is awful. Except the above set, it MAY work, but not sure.
Aromatisse is not better, abilities wise. Sure, it gets two near-useless abilities, in single, half decent in double. You should be using in either fact Aroma veil, to support Outrage Scarfchomp and some rare Petal Dance and Trash users. Stats wise, it is not as evened as Slurpuff, it has an excellent 101 base HP, but a very very baddass speed. You should make a use of it, in a trick room team. This page should help you finding movesets.
3: The choice
2 mediocre Pokémons means no choices, if possible. Try to get a Florges, it is better and by far. But if you shall stick to one of the two, I would pick Aromatisse if you play a Hail Room team or double battles, which will be quite popular with Mega-Abomasnow speed reduced. Otherwise, pick Slurpuff for single play.