I just want to say London out of US-hate/frustration, but I honestly think that New York is the better of the two. Of course, the two differ only slightly, but I don't think that New York is coloured any worse because of my American stereotype, because it really is of a class of it's own.
I find that, compared to London, New York is a lot nicer to start with. Especially after 9/11, most people are very friendly to eachother, in a way that the centuries-old classism and division in London just don't pull off. In my opinion, New York is a fine example of that Canadian cultural mosaic, and that's a pretty big thing that makes the city, imo. Of course, maybe London just needs a plane to fly into Buckingham Palace or something and then it'll be just as friendly and accepting as New York.
Though the two really are pretty evenly tied. It's like 1001 vs. 1003 for me, really. But I think Tokyo should be an option too, also being considered as one of the really important global cities, and also maybe just because of the fact that it has a higher population than all of Canada. Then things would really be squaring off in terms of preference.