I think that Seoul maybe better than HK.
Although, Seoul maybe not better in terms of expat community and language, but overall I would recommend Seoul.
The scene is improving considerable these days here in Korea (even gay-themed TV commercials.. )
Itaewon is more "western" color while Chongno is more "Korean" color. College students and yougsters gather at Shinchon... etc. etc.
You can also experience history, palaces, exciting pop culture, etc.etc.

If you need more help, PM me.