I think the major difference is that you do not get to see the same doctor everytime if you are going for check-ups in a public hospital. In general, the doctors are pretty good there. I've forgotton the charges in the hospital for check-ups, but compare to private OB/gyn (avg. 450 - 600 HKD per visit), you can save a lot of money here.
I was registered at Queen Mary (for private) pre-natal package, but they have dropped all the package deals last year. I was very happy with my doctor there (Dr. K.Y. Leung), but I switched to a private hospital for delivery at the end because of the changes of the charging scheme.
Congratualtions & good luck!!
Cyndi