I agree with jpatokal that pollution in Hong Kong can be, indeed generally is, worse than you see in Bangkok. The nearly constant haze emanating from mainland factories blankets Victoria Harbour and fairly well destroys the view, not to mention what it does to those with breathing problems. Over the last seven years I've been visiting Bangkok, air pollution has improved dramatically. Maybe one in three days in Bangkok are crystal clear now, and the rest are rarely very bad. You'll only get the old feel of Bangkok pollution if you're directly behind a heavily polluting vehicle, be it a bus or a truck or whatever. Thai authorities deserve great credit for cleaning up Bangkok's air over the last 10-15 years since the first serious pollution-control regulations were adopted.

That said, Bangkok still can be a dirty city. It's not so much air pollution as urban grime and refuse, much like you'd find in New Orleans or New York.

Assuming it'll be the OP's first time to either HK or S'pore, all things being equal, I'd suggest HK for the more exotic experience. But if the OP has only two days and must spend some of the time in S'pore, I'd say spend all of the time there to get a proper feel for the city. There is tons to do. Not least, eating. And not just in the Hawker Centers.