Having lived in both Hong Kong and Singapore (and since those two cities are frequent object of comparison), here is my unique (and totally biased and over-generalized) take on living in both cities:
1. Singapore has better urban planning than HK in terms of more open green space, but those green space is more of an aritficial and manicured creations. Whereas in Hong Kong, while urban spaces are more cramped and concrete over, there is a more real sense of "wildness" in our country parks (which we have more than in Singapore) You can enjoy hiking in Hong Kong more than in Singapore.
2. Singaporeans tend to be more polite and friendly than Hong Kongers , but Singaporeans also tend to be more inflexible and rigid in their thinking than Hong Kongers. HK folks tend to be more rude, but also more "street-wise" and more entrepreneurial. There are exceptions of course, but those are general observations.
3. And less subjectively, (and also very ironically), you enjoy more real freedoms in Hong kong (which is not a democracy) than in Singapore (which technically is a democracy...in theory), where the government is very much a parental overlord.
4. No question about it, Singapore has less pollution than Hong Kong. Except when you have those Indonesian wildfires blowing over...
5. Hong Kong is bigger geographically and demographically than Singapore, which means there are more things to see and do...or is there?
6. HK summers can be unbearably hot and humid, but it's not year-round hot like tropical Singapore. Since HK is more subtropical/temperate than Singapore, winters here can be pleasant, temperature-wise.
But the above are only my personal take, how do you see the comparison between Hong Kong and Singapore?