I'm impressed by neither, to be honest; not my kind of music. Logic however dictates that Vanessa Carlton has more talent, if only because she doesn't need two *cough* "ghostwriters"
to write songs (which Avril does... And then she/they still come up with the incredibly lame lines
"She was a girl/He was a boy/Can it be any more obvious?"); and Vanessa actually knows how to play an instrument, which Avril Lavigne only *claims* that she can. There are a lot of pictures available of Avril playing guitar, but from what I've heard, she never
actually plays guitar during concerts except for some simple chords.
Voice wise... Vanessa Carlton lisps, sort of. Avril's voice is okay, nothing special however.