We just moved to Hong Kong from 1 year post in Beijing. I would say Hong Kong is 1000 times better than Beijing. We lived in Beijing for 1 year and 4 months, to a point that we were counting days to the airplane departure from Beijing. And we were willing to drop everything, breakup apartment lease, etc. to get the hell out of there.
To name the few, air pollution , water pollution, noisy pollution from construction and traffic, crowded population, poisoned food, fake products, bad service, crazy driving cars, controled media and internet, etc.
local food and goods are cheap, but the quality is very uncertain. I bought imported goods most of the time, but they are very expensive. I have to pay the extra for the peace of mind.
forget about outdoor sports, unless you want to play tennis in air pollution. picture that you breath lots of air with heavy metal/dust while you exercise. That would cancel out any health benefit you might get from exercise. When I live there, I only do indoor exercise in gym. It makes me miss California so much.
The only good thing in Beijing is the housing cost is half or even the third of housing cost in Hong Kong. For half of the rent, I can live in a very nice apartment in a very nice area.
I visited Beijing for few days before we move there. It was fun to go sightseeing in Beijing. But when we moved there, lived there, it is completely a different story.
If you have further question about living in Beijing, you can ask me.