I don't remember much of Singapore, so no comment.
At this time of year, I pick Hong Kong. Japan is freezing me to death... HK will be cold, but not freezing.
HK has a lot of otaku hobby related things at a relatively cheaper price compared to Japan, which was a merit when I was living there.
As for Japan, if you are prepared for the cold of mid-Feb, and have the money, will satisfy your needs for almost everything. Plus being a larger place than HK means that you have more places to explore.