HK beats Singapore hands down for me - better weather and better hiking.
But if you like eating, Singapore wins imho. And I hear flats are cheaper too.
HK is better for public transport (but both beat the UK hands down!).
OP: I was in a similar situation as yourself and chose Hong Kong because of the perception that Singapore is too boring. Two years later and many back and forth trips to Singapore for both work and pleasure I still think that Singapore is quite boring.
As others have mentioned everything is quite artificial. The social scene is bit slower than HK and the dating scene is abysmal according to my friends. Their primary complaint is that ex-pat social circles are so intertwined that you'll inevitably end up dating the ex of a friend or seeing ex's at bars/clubs on a regular basis. Although HK has the same issue to a lesser extent.
That being said, the overall quality of life in Singapore is much better - cleaner air and streets, much better and cheaper food options, and generally people are much nicer / friendlier.
Singapore: Asia's Word City??? With press freedom at 154 out of 180, I don't think so. Freedom of speech and assembly is also lacking.
Takes me a few days to get used to places where people are just a bit nicer than HK. First day or two, I'm like W(hy)TF is someone looking at me and smiling, do I know them? What scam are they running on me!?
But then you get used to the other place just being nice... And then you come back and have to deal with fucktard taxi drivers who do not want to get out and help you put your bags in the trunk. In 2 mins, you're back in HK mode - fuck the world, now lets go make some money.