To answer your questions, I own both Filco and Archiss Progresstouch (basically a rebranded FC900R from the same OEM). In fact I'm typing this from the Archiss keyboard now.
To rank the stabilizers strictly on feel, I'd say Costar > Clipped Cherry (Yes those grey stems are clipped Cherry) > Regular Cherry.
Clipped Cherry already feels really good but you can still feel the friction from the stabilizer sliders a tiny bit, it goes away with some use or you can also lube it.
BUT if your goal is to get thick PBT keycaps. Then I'd just get the Leopold. There are 2 reasons.
1) Costar stabilizers are known to grind into thick PBT keycaps. The metal bar doesn't have enough clearance when fully bottom out. More notable on Shift and Enter keys.
2) PBT keycaps on its own already cost around $75. They come stock with Leopold so you save a lot of money going that route.
And ultimately I do think Leopold has better build quality than Filco (Which is already super amazing, but Leopold just edge over by a tiny bit). So in your situation I'd highly recommend the Leopold.