Short list (in order of my personal preferece):
-PBT caps
-Shortcut keys
-Media keys
-Removable cable
The PBT caps kick ass, hands down. If you already have a Filco, you could just grab a set from Imsto's group order. The other pros are a little more subjective. I, for one, really like the shortcut keys as am constantly bringing up the calculator and "Computer" in Windows and they make doing so convenient without grabbing the mouse or setting up shortcuts in AutoHotkey. The media keys are cool, too, but a little inconvenient since they are second-layer keys, but they are still there and other 'boards don't have them. The removable cable is cool, too, but not really all that useful unless you have other keyboards that you swap around with removable cables as well. What I really like about having a removable cable for, though, is replacing the cable with one of an appropriate length for my desk, but that's more about aesthetics than function.
The Leopold has a removable cable and PBT caps, but lacks the other features, and the Filco has none of that, but you can get the PBT caps from the group buy. That said, you can just buy the 9008 and get all of it in one package. I would suggest spending the extra cash and getting a G2 version with the engraved caps, but the standard 9008 is nice, too (I am using the OCN edition right now). Unfortunately, they don't have a tenkeyless in the 9008 quality tier, so this really only applies to those that like full-size 'boards like me.