Filco:
TKL
Thin ABS keycaps
Costar stabilisers
Fixed cable
Ducky:
Full size
Thick PBT caps
Cherry stabilisers
Removable cable
A lot of the differences will come down to preference. I prefer:
TKL - Allows your mouse to be closer to the main typing area of the keyboard and brings the main typing area closer to the centre of the screen.
thick PBT caps - more durable, wears slower, much better feeling due to the weight of the caps
Cherry stabilisers - easier to change keycaps, less rattly, less fiddly (no little inserts), compatible with more spacebars (Costar can stick on spacebars with thick material or different interior angles)
removable cable - so I can uplug the cable at the keyboard instead of at the computer, mostly useful for smaller keyboards
So the only thing that attracts me to the Filco is the TKL size.
The keycaps on both are standard layout, so you will be able to replace them with any aftermarket keycap sets. If you don't mind the full size of the Ducky and you don't have extra to spend on keycaps yet, buy the Ducky.