Between the three you mentioned, the key difference is the stabilisers (calling them stabs from now on...) each board uses.
Both the Ducky and the Leopold use Cherry stabs, whereas the Filco uses Costar stabs.
Generally speaking, people prefer the feel of the Costar stabs, myself included.
For the first keyboard you buy, on the other hand, you aren't going to know the difference.
So, in your case, I'd recommend buying the Leopold - I've used an FC500R with MX Browns quite a bit, and its a very good keyboard.
Later on down the track, if you feel like you want to spend a bit more money, by all means get the Filco.
There is also a reported difference in the quality between the Filco and the Leopold, but that all depends on who you talk to. There have been many threads arguing over this, but the general concensus is that Filco are of a higher quality, but the Leo (and Ducky) are still excellent keyboards. At least, having tried all three myself, thats the conclusion I've come to.
tl;dr: Got plenty of money, get the Filco. If you're on a budget, either of the other two are fine choices as well.