If you are doing business in China, then Beijing, Shanghai, or Shenzhen would be your best options. You really can't do a business for China from HK effectively, unless part of the business is connecting with the rest of the world in some way.
That being said, big differences are the ease in setting up a company and costs. Ease of getting visas and the general paperwork. Also, HK is much safer than any of the mainland cities. The big cities are probably about similar to New York in terms of safety, but Hong Kong is ridiculously safe in comparison. There are basically no areas that you really shouldn't go to, and the medical system is quite good and reliable vs. mainland.
Also, one nice feature, no corporate tax if the income is from outside of Hong Kong.
That being said, big differences are the ease in setting up a company and costs. Ease of getting visas and the general paperwork. Also, HK is much safer than any of the mainland cities. The big cities are probably about similar to New York in terms of safety, but Hong Kong is ridiculously safe in comparison. There are basically no areas that you really shouldn't go to, and the medical system is quite good and reliable vs. mainland.
Also, one nice feature, no corporate tax if the income is from outside of Hong Kong.