Singapore is a bit better for culture, although HK seems to be picking up a bit in the last year or so - getting back on the circuit for bigger name bands, more English-langauge professional theatre and so on.
Both Singapore & HK have a good range of bars & restaurants. I can't speak for the club scene - not my thing.
Singapore is a bit cleaner as an environment, but some would say to the extent that it is "sterile". Both cities are far cleaner than London. Hong Kong has a bit of an air pollution problem, but personally, although it needs fixing, I feel people exagerrate the harmful effects - to me it is more aesthetic at the moment.
Singapore is also essentially a dictatorship run by one family, which means that you may be uncomfortable with the lack of freedom of speech and other restrictions there. If you are gay then HK is clearly the place to be. If not then in either place, if you are white, you'll find it easy to have as much fun as you want
Housing in Hong Kong is expensive. If your employer doesn't provide it then you'll need a bit more net income in HK than in Singapore to allow for that.
Singapore enables qualified foreigners to apply for permanent residence within 2-3 years which means never needing to worry about a work permit there. In Hong Kong you need to be ordinarily resident for a continuous period of 7 years.
Tax is much the same (and low) in both places. At the very high end (US$300K+ I think) Singapore's is higher, but both are way below European levels.
Where HK really scores over Singapore is the countryside - HK is 40% protected country parks, with 3000' hills and excellent hiking trails. Even if you live in the heart of Central you can be on a hiking trail in 20 minutes, and with a big dog you might choose to live in the countryside on Lantau or out near Sai Kung and you would still probably have a far shorter commute to work than you do in London. And the public transport in HK is superb.
English is more widely understood in Singapore these days - in HK Mandarin is now clearly the second language, with English an increasingly distant third, but I guess that becomes an irritation for me once or twice a year and is never an insurmountable problem.
Seven years ago I had the same choice as you. For me then the choice was easy - Hong Kong. Now a few factors have moved in Singapore's favour: English, pollution & (at a superifical level at least) a relaxing of some of Singapore's more extreme restrictions. In summary, today I think I would still probably make the same choice all other things being equal, but if the incentives were good enough Singapore isn't out of the question any more.