I was born and raised in New York, so I'll probably always love it more. That said, I live in London now, and I really do love it. I am a city girl, it's my comfort zone. For those who have been talking about the smells in either city, the reality is most cities are always going to have a bit of a funk. There are a lot of people, a lot of cars, etc, and that's just going to mean there will be a lot of smells! To me, it's just part of the charm of a city, I guess. I've seen no more vomit, etc in London than I would see in Manhattan.
My experiences here so far have been largely positive - I can't complain about the people. (I mean, I can, but it wouldn't be a London people complaint it would just be a people in general complaint!). The people I've met in the city have mostly been very intelligent, friendly, and liberal, so we've had a fair amount to talk about. Also, I think I probably have an easier time of it talking to people because I think we share a similar city mentality.
I do think both cities are an acquired taste, though. I have no other basis of comparison, as I've never lived anywhere but New York (well, Albany for four years during college). But I know people who were not from New York or a city didn't love it. Loved to visit, but not to live. As my friend once said "Just because you can fit all those people in a city doesn't mean you should!" and I guess there's something to be said for having more space. Me, I like getting lost in a crowd, and both cities let me wander around to my heart's content.
I don't think it has to be an either/or situation, either. Just because you prefer one, doensn't mean the other sucks, right?