Thanks Guys.
Eventually, I have choosen RF 87U for couple of reasons:
1. HHKB's Fn key's position
Even it can be placed on left meta key, it still might be an issue for me because the right meta key won't changed.
As I said, my right little figure is not as agile as my left one.
2. Function keys and Windows Keys
As a developer, I have to spend a lot of time to read the code not only write my little creepy things.
I need the function keys which can do lots of help on Visual Studio, like F12, CTRL+F3..
It will be a burden if we have to press one more key, especially while using mouse.
And it is also the reason why I didn't choose FC660C.
3. This keyboard is mainly used for my works.
I won't take it home and use it on my laptop. So there is no necessary to get used to HHKB at work, and switch back at home.
So, this is my choice.
Anyway, thanks for all of the suggestions.