My first instinct after seeing both was instantly, "Rarity." Could just be that I like purple, heh heh. The horn is definitely a plus, costume & cuteness wise...why doesn't the Fluttershy costume have wings? ^_-
In the one shot where the person wearing Fluttershy has the sleeves up over her shoulders rather than at the sides I actually think that "dress" is the cutest (really not a fan of it with the sleeves down--weird how that one detail make such a difference), but still I think the Rarity one looks more costumey because of the horn.
Update: Whoops, I was too late, heh heh. Fluttershy IS cute though; just make sure to wear the sleeves up!
(LOL, jk, wear it however you want)