I'm an Canadian born Chinese who has both HK and Mainland friends (both guys and girls). Based on my experience, both Chinese girls are very materialistic and care about looks a lot but HK girls seem to be more materialistic. However, I prefer HK girl friends because we tend to get along better and have more similarities and interests whereas Mainland girls, based on my experience, are a lot more "smarter" in a bad way (deceiving), and tend to hide a lot of things to themselves. As such, I find HK girls more honest and speak their mind, a trait I admire in friendships.
I also asked them the difference between HK/Mainland girls and HK friends say Mainland fashion is bad/old-fashioned (direct canto translation: leurng) and that Mainland girls are full of themselves and think they're really pretty. IMO (based on the Chinese pop'n in my University), I can't really see a difference other than that Mainland/mandarin girls' fashion seem to be more influenced by Korean/cute style and HK girls wear more "mature"/sophisticated.
Oh and these "Chinese" girls I'm referring to are mostly international students that came to study in Canada so I can't speak for the ones living back home.
Just some things I've noticed.