I have not used an FC660C, but if they had been available when the opportunity presented itself to buy my HHKB, I probably would have gone with the FC660C.
As it is, I ended up with the HHKB, and haven't regretted it.
Due to the FC660C plate-mount vs. HHKB case-mount, the FC660C is usually reported as feeling heavier and more solid. Also the sound is different, with the HHKB usually being reported as having the more representative Topre "thock" sound (except for the silenced HHKB).
If you end up a Topre fan, chances are you will end up with both keyboards anyway at some stage. The only thing stopping me from getting an FC660C is lack of funds.