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There's some overlap, but "you wish" can only be used to contradict the listener's desire, "as if" can be used to contradict anything they say.

Still sounds vaguely teenage girly though.

It's used as a contradiction of or challenge to whatever has just been said, as much as a statement of doubt.

"You expect me to wear a suit? As if!" (meaning "no, I won't wear a suit!")

"He...