I would bring both. Whenever I go out on the town with my baby we usually take both. I would collapse if I had to carry my baby around all day.
Lightweight is right - none of those crazy western travel systems. I was shocked when I went back home recently and saw those monstrosities.
If you wait for people to help you, you`ll have to wait a long time. I always enlist the help of people. I have never been turned down - I feel like they`re glad I asked
You could look into renting one - I think the YWCA (near Central/Mid-Levels) rents baby things out for a reasonable fee. You can check out their website, its under "other services" (google Hong Kong YWCA English Speaking Members for the English site).
Would recommend bringing one with you. Strollers and baby products are not as reasonable priced as the US, and you will probably use it at the airport as well.