Difficult choice actually. I'm a West Coast Canadian and SF is somewhat familiar in that regard, but different too - so, quite seductive. Then, New York is the kind of big city I'm totally addicted to. Fascinating variety, incredible energy, history and culture and sleaze and glamour and art on every corner.
I'd probably have to go for NY. When I think about cities I'd still like to give a try, NY comes to mind much sooner than SF.
new york is cool to visit, but it's kinda.. soulless to me. i don't know if i'd ever want to live there. plus i don't like how a lot of new yorkers act like manhattan is the center of the universe and people from anywhere else are hicks.
i'd rather live in oakland than san francisco though, but that's probably just me.