I have both.
My HHKB is hypersphere'd and my 660 is stock.
As others here have said, the 660 is snappier and feels more solid, likely because the 660 has a metal plate while the HHKB is all plastic inside and out.
There's essentially 3 embodiments of Topre: HHKB, Leopold (FC660C, FC980C), and RealForce (55g, 45g uniform, variable). And there's people who change the domes on various boards.
I think the choice of which keyboard to get really depends on how you're going to use them. If you use arrows a lot, you may want to get the 660 because always having to access arrows via a modifier can be troublesome sometimes. If you're fine without them, then I think its a matter of preference.
Personally, I favor my HHKB because the default layout and dip switches on the 660 annoy me. I prefer the back space being on top of the Return key, and I've swapped the Caps lock and the left Control with the dip switches, but I keep accidentally hitting the new Capslock now. I want absolutely no Capslock on my board. Rather than getting a Hasu, I'm probably going to sell it.
Other than the layout, I still prefer my HHKB, because when Hypersphere'd, or if you get the Type-S, it's likely the quietest keyboard. It's even quieter than the Apple chiclet keyboards all my coworkers use. I prefer the softness and the hallow thocky sound of the HHKB more as well.