Quincy makes me think of the quince fruit, and
John Quincy Adams. I have a hard time imagining it on a real baby. For me, it has a kind of pretentious/uppity feel. I think
Quinn sounds a lot more down-to-earth. Or perhaps
Quentin or
Quintin.
But I agree with you,
Quinn seems to be used more and more for females. When I hear the name, I think of "Dr.
Quinn, Medicine Woman" or the sister
Quinn on the show "
Daria." Okay, my references are a bit dated but even still-- I can only think of female bearers of the name in the media.