Obviously lots of discussions on this one already but just to give u a few views from someone recently settled in HK but previously lived in Singapore for 6 years. so, I am no expert on HK but I do know my way around Singapore. My initial observations are the following:
In terms of language, HK is not as English friendly as compared to Singapore. Cantonese, followed by Mandarin are the most commonly spoken languages but as long as you do not venture into the remote regions, English speaking people are available. Compared to Singapore however, you can tell that many are not so comfortable speaking English. (purely based on my observation - staying near Yuen Long area so far away from HK city center)
The climate is quite different. Singapore only has 2 seasons, the rainy season (temperatures vary between 24 and 32 C and it rains a lot), the dry season (temperatures vary between 24 and 32 C and it does not rain so much). If you can handle the combination of heat and humidity, you will be fine. Suggest to spend a few weeks there to see first because I have seen people decide they cannot take it after staying there for a month. Hong Kong (as far as I can tell) has more seasonality; right now it is fairly cool (between 10 - 20 C) but the summer can be just as hot and humid as Singapore.
There are similarities as well, both places have excellent public transport systems and (as far as I can tell) quite efficient governmental services.
Owning and driving a car is definitely more expensive in Singapore - right now the costs are going thru the roof, but in HK I understand that parking is the main problem.
OK, that's about what I can share from my initial experiences, I hope it helps. Just my 2 cents trying to help out where others have helped me out here also ;-)