I have both the 660 and type s, they are similar for the most part since they're both 45g uniform. The main differences are:
- 660 has a deeper throw, HHKB feels shorter
- The bottom out feel is softer on the 660, more forgiving
- 660 is quieter than HHKB but louder than type s variants
- Layout differences, they're pretty different as far as mini layouts go. It's all personal preference though
- Price difference, varies depending on where/when/how you get it