I have a Filco w/ browns I've been using for over a year and a Leopold with browns that I have very low miles on. Nunez has said quite a few of the differences already, I'll just augment a little.
- stabilizers are a significant difference. Which switch you use will probably affect how much you notice the difference. From what I've read Cherry stabilizers seem to work best with stiffer black switches than with lighter reds or browns. I've used terminal and Amiga keyboards with Cherry stabilizers for years (before I realized what I was using) and loved them. When PROPERLY lubricated (not over, not under, and not with grease that is too thick) I think the Cherry stabilizers are just fine on the lighter switches too.
- If memory serves correct, Leopold does a trick to get 18KRO over USB. I think it appears as three keyboards or something like that - 6KRO each. I believe that this "trick" makes the keyboard incompatible with some platforms. I haven't had a problem yet, but I've read about it.
- The detachable cable is a plus and minus. If you set up your work space and more or less leave it alone I think detachable is a plus. You can go to monoprice or similar cable retailer and get the length of USB cable you need for your installation (from 1' to 16') and have superior cable management and tidiness in your workspace. If you carry your keyboard around with you all the time and are constantly connecting, disconnecting and moving it, you might have some service life durability issues compared to a hardwired board.
- The Leos have LOCK button LEDs integrated into the keys. Ordinarily this is cool, until you want to fit third party keys that don't have windows for the lights. Then you have no lock lights. As cool as in-key lights are, I think that separate on-board lights (like Filco) are a better option for enthusiasts likely to change their keys. Unless of course you use an tray app that displays your lock status or don't care etc...
- From my (so far) limited experience with my Leo, build quality seems similar to Filco. I believe that Leo stock keycaps may be better than Filco.
- There's posts here about controller bugs on the Leo. I've seen the posts but haven't paid close attention nor have I experienced a real problem. I don't know if some of these problems are related to the 18KRO thing. I'm also not sure if both TKL and full size are affected by the same problem.
- Leo's are a little easier to get and EK has lots of switch options. I don't think I've ever seen a Filco with clears that wasn't a mod, for instance.