Fellaini came here and was... Felliani. We got what we paid for and he played to the best of his ability, which is obviously nowhere near the level of a proper United player.
Mata came here and was expected to be a superstar, just as he was at Chelsea and hasn't come close to living up to those expectations.
Mata is aesthetically pleasing, a nice guy and has Juanfield; there's no way he will lose a poll to the antithesis of this.
Felliani represents a lot of what nobody wants United to be: hoofball, donkeyish, the bad side of physical etc. Mata, at his best, is a player many would hope could slot into a United side playing great football. Again, Felliani stands no chance in a poll when this is considered.
I'm no fan of Felliani, but he is the winner of the two for meeting expectation of the signing.