I was in your position last year, but I lived in Shenzhen. I looked at Union hospital and after a lot of thought, we decided to have our baby at the VIP Birthing Centre in Guangzhou. It was basically to avoid having to stay in HK before and after the birth (to get passport, visa to go back into china) and the high costs associated. I did, however, go to Union for a few check ups, notably the fetal anamoly scan.
We were both happy with our choice and everything turned out well (well enough considering everything turned out unexpected in my birth anyway, but that`s not related to my choice of hospital). If you want any other details, please PM me.
Oh, and I`m sure you`re already in the know, but just steer clear of SZ hospitals. 3rd world.