.com.hk is not more 'popular' (if you define that as meaning more numerous) than .hk, it's simply that the .hk extension was introduced much later, and now accounts for more registrations each month. One reason would be because the eligibility rules are less restrictive, but I've also observed in the past year that more companies are choosing to use .hk
A HK-based company might use .com, .net, .com.hk or .hk, and most of them appear to be unconcerned about which they choose. The use of hyphenation also appears popular, so you can say that most are not SEO-savvy.
I've not seen an aftermarket for .hk names. I own several generics which would fetch xx,xxx in the .com extension, but when I parked them the traffic was negligible, and the hits I got most likely other domainers since I've yet to find anybody here - local or expat - who defaults to type-in rather than search engine. Given the chance that what they're seeking in HK could be appended with one of four different extensions, you can understand why.
So, my view is that in the short to medium-term, .hk is probably only valuable if you intend to develop a HK-centric site.
