They both have non-standard spacebar stem positions (the switch is in the correct position, but the stabilisers are closer in than standard) which means very few aftermarket keycap sets will fit. Funny thing is that they have made their spacebars with extra stem positions, so their keycaps will fit any keyboard, just not the other way around.
At least the keycaps on the 750R are very nice, though, thick Cherry profile PBT.
If you don't mind the limited number of keycap sets you'll be able to fit on them, they're both very nice, very well-made keyboards and it just comes down to whether you need dedicated F keys or not and how important the size is to you.
There isn't really an exact alternative to the FC660M since it has a unique layout (and it's the best compact layout with dedicated arrow keys that I am aware of), but Filco and Ducky make very good TKL size boards which are of similar quality to the 750R, although with worse standard keycaps.