I would add my support to PDLM's post, everything in that was right on the money.
I did have the choice of living in any city in the world, and right now chose Hong Kong. I've been to Singapore several times. I'm happy with my choice of HK, I think HK is still a clear winner over Singapore but less of a winner than it was some years back.
I am assuming you don't speak Chinese. If you speak Mandarin that would probably make Singapore the winner for you.
Singapore is quite claustrophobic and very clean to the point of bordering on sterile. Hong Kong feels more like "anything is possible" which is a feeling I like. Lots to do here, and to me people seem more open and welcoming than in SG.
The big downside of HK is the air pollution. But you probably won't feel the effects on your skin for quite a few weeks after you arrive. I don't know how bad your breathing problems are, if they are very bad it may be a cause for concern. Personally I find it effects my skin not my breathing.
Do not get involved with illicit drugs in Singapore. Do not be in a room that has illicit drugs in it in Singapore. Period. If you find yourself in that situation, get out of the situation immediately, no matter what the consequences. There can be no consequences worse than being hanged from a noose until you are dead.