Do you speak/read Shanghainese, Mandarin, or Cantonese. If neither of those, I suggest you stick with Singapore because you have two kids and due to school, space, and pollution.
here's my very long 2 cents.
About HK....
Schools in HK have a very long waiting list (i.e. ESF and
International Schools unless you are willing to fork out $ for debentures, then local school is the only way to go). Also, schools start at age 2. (i.e. Kindergarten are N1=age2, K1=age3, K2=age4, K3=age5 in HK). While not required to attend school until age 5, there are no nursery for age 2, only kindergartens. I tried sticking my 2 year old in a local kindergarten. The teacher didn't speak any English, my child and husband didn't speak any Chinese. Example, the homework instructions wrote, "color the animals only", so if you couldn't read it, how do you know what to color....how's the teacher going to explain to you every homework?
Also, in HK the costs per square footage is higher. Also, the current square footage published is not exact. Example, you can see a listing for about $3000USD for 1000 sq ft, but when you arrive to see it the agent then tells you the efficiency level is 80% (meaning really just 800 sq ft). This is because 2000 sq ft is the lobby shared by 10 units, thus each unit adds 200 sq ft)...crazy, huh?
In Shanghai, there are Mandarin and Shanghainese (you can get by with one of these, if none, then much harder). Rent, food, etc...are the cheapest, but then can you stand the etiquette in China? Let's just say that downtown is fine, but just outside of the office, I lose count of how many people I see spitting, coughing up phlegm, and cutting in line.
Cleaniness
Shanghai Dirty (outside of downtown)
Hong Kong Dirty (outside of downtown)
Singapore Clean
Food
Shanghai Cheapest (mostly Chinese)
Hong Kong Cheaper (has good mix of Chinese and Western food, not so much Malay food, etc)
Singapore Cheap (good mix of cultures of food)
Pollution
Shanghai Terrible
Hong Kong Somewhat bad, sometimes good
Singapore Not a problem
English Level of Locals
Shanghai Not very good, limited areas only
Hong Kong In downtown is good, some expat area good, outside not so much
Singapore Good with Singlish
Price of housing
Shanghai Cheapest in terms of square footage
Hong Kong Most expensive in square footage
Singapore Middle
Schools
Shanghai Depends where you go, unless you want all Mandarin or Shanghainese, Int'l School is the only way
Hong Kong Most difficult to get into
International School or Bilingual Schools, waitlist is absurd, local school is really local...no instructions in English.
Singapore Ok
Weather
Shanghai not sure, didn't stay for all 4 seasons
Hong Kong Has 4 seasons, except winter is cool not cold. (similar to Seattle----gloomy about 20 days in a month, then 10 good days of Miami sunshine).
Singapore Hot and humid mostly
Public Transit
Shanghai OK (still better than public transit in the states)
Hong Kong MTR is among the best, with lots of connecting buses (cars still affordable..so taxi is cheap)
Singapore MRT is good but breaks down sometimes