I would recommend buying your H in Tokyo, either at any of the H stores or one of the larger H specialty resellers. While I am sure that there are some options in HK, I can tell you that the selection in Tokyo is far, far greater than anywhere else in the world and the shopping experience in Japan at both the H stores and the resellers is fantastic!
In other words, unless you find EXACTLY what you want in HK (and are sure of its authenticity if you buy from a reseller), it might be wise to wait for Tokyo.
If you want an exotic, there were many ostrich and croc bags at the small, appointment only, H store at the Ritz Hotel (? If forget, but I think it is in the Maronuchi area) when I was there.
If I were you, and if you are looking primarily for Birkins or Kellys, I'd first try the Ginza H, then the Ritz Hotel one, and then go on to some of the respected resellers. The H at the Tokyo department stores were very nice (and the SA's at all wonderful), but didn't seem to have the same, large selection of Birkins. (Locals, correct me if I'm wrong!)
Good luck!