I am not familar all these rings and diamonds but....
Pros for buying in HK
1. No Tax
2. More Choice (from Tiffany to Chow Tai Fok / Chow Sang Sang which are famous brands in Chinese Community, all have quaility products)
Cons for buying in HK
1. Credit card charge / Tax
2. Limited time to select
3. Warranty Issues
So I guess I try check out the HK store first, if nothing really matches, fail back to US option. There are plenty of shops around TST / Mong Kok / Causeway for local brands and Central for the upper ones.