HHKB Pro 2 Type-S is superior to the FC660C. I have both.
I don't have the regular HHKB Pro 2, but most of the following will apply to that one as well.
Here is why I prefer HHKB Pro 2 Type-S:
1. Layout of the keys. HHKB has distinctive key placement. Control, backspace, and ~, and pipe bar |\ are all in HHKB pattern. Which is better.
2. Sound of the HHKB Pro 2 Type-S is better.
3. The look of the white and grey is better than beige on black of the FC660C. Somebody above said it was white on black, but it's not, it's really a beige on black. Possibly even it's supposed to be a little golden beige or something. It looks like crap, quite frankly. Although I really like the keyboard, I really don't like the color scheme.
4. Blanks are better than letters.
BUT, the FC660C has one great feature and that is the price. At under $200, it's very appealing. However, the HHKB Pro 2 is better. If you get the HHKB Pro 2, you will have one keyboard, since there will never really be a reason to buy the FC660C. On the other hand, if you get the FC660C, eventually you will have two keyboards, because eventually you will buy the HHKB Pro 2 as well. There is no better way I can explain it. Of course having moar keyboards is better than less, but you might want to save money and just get the HHKB Pro 2.
Thanks for listing your reasons for preferring the HHKB Pro 2 Silent Version over the Leopold FC660C.
For many of us, the layout of the HHKB Pro 2 would be the deal-breaker. For some, the layout of the HHKB Pro 2 is ideal, but others need dedicated arrow keys in the primary layer and a standard layout in the main typing area. This is why the layout of the FC660C is so appealing -- it has dedicated arrow keys and a nearly standard layout in a 60+% form factor.
It seems that there are many who agree that the keycaps on the FC660C leave much to be desired. They appear to be laser-etched with an infill of yellowish gold lettering on a chocolate brown background, which matches the case. To me, the keycaps and case on the FC660C look more brown than black, whereas the case on the FC660M looks black in comparison. Being made of PBT, the keycaps feel good, but the color scheme and method of printing look bad. If the FC660C had the same dye-sub PBT keycaps as those on the white/gray versions of the HHKB Pro 2 or the Realforce 87U/104U, then it would be a winner. Even for those who like blank keycaps, the background color scheme ought to be like that on the white/gray HHKB Pro 2 or the RF 87u/104u.
As for price, relative to other Topre-switch keyboards, the FC660C is attractively priced at 189 USD from EK, but I would submit that all of the Topre-switch boards are overpriced. On the other hand, if I were to purchase a HHKB Pro 2 or RF87u in the white/gray color scheme, I would feel no need to put sets of custom keycaps on them or to put them into custom aluminum cases. By comparison, a customized Poker II would end up costing over 300 USD, and with its Cherry switches, it would still be inferior to any Topre-switch board for typing, IMO.
Like many people, I greatly admire the elegant good looks and compact form factor of the HHKB Pro 2, but I fear that I would find its unique layout and absence of dedicated arrow keys too problematic to be useful.
It would be great if PFU and/or Leopold would come out with sequels that address issues that customers and prospective customers have identified. They might want to continue offering their current models for those who find them entirely satisfactory. In addition, or alternatively, it would be fantastic if Realforce were to produce a new keyboard in the 60+% category that would essentially be a refined Leopold FC660C or hybrid HHKB Pro 2. Either way, I think that customers would pay hefty prices for such keyboards and actually be satisfied with their purchases.