1. If you order online (e.g. BHphotovideo) and pay no sales tax, US is cheaper.
2. If you go to the store, and pay sales tax, HK is cheaper.
You also have to consider warranty condition. In US you can get grey market goods which costs even less but you will find it difficult to have them serviced in HK.
You can check the HK prices from these forums and make informed decisions: