I've lived in both now and find them similar but quite different. Hong Kong felt like more of an adventure, being farther away from all that I knew and with a much more limited pool of colleagues and friends, but it was also a lot easier to get around and do things when I wanted to go out. New York beats HK on cultural opportunities, such as the theater and museums and the like, and I have to say you will find the overall culture a bit easier to adjust to than HK. Both have substantial UK expat communities, but as another poster mentioned, the underlying community in the US is English speaking and in NY not all that different from London (they call those who live on the flights back and for NyLons). In HK, the underlying community doesn't speak your language, eat your food, or have the same background growing up. It can be a bit of a shock after a few months. You will meet a fair number more Aussies and New Zealanders in HK than NY (they are more spread out in NY).
But with that said, I still revert to my basic advice on overseas travel for work: where is it more likely you will end up in the future? If this is realistically your only chance to live/work in HK, then you should consider it. I think, given the distances and closeness of NY/Lon, you'll probably have a chance to live/work in NY again, but now, in HK, might be your only real chance to experience it.
I once met an 18-year old Marine fresh off the cornfields and just out of boot camp flying home to see his folks. He was from Missouri and wasn't sure where he would be posted. His choices were Arkansas (next to Missouri) or Italy. I had to sort of talk it out with him but basically my advice was "son, you have the rest of your life to live in Arkansas. This is probably your only chance to live expenses paid in Europe." He really had never thought of it that way, but in the end, he realized what I was saying and called his commander from the airport with a request to go to Italy.
You will probably have a chance to go to both HK and NY in the future (I'm not sure) but you might want to think of it along the lines of this young Marine and see if that helps your decision.