I have the wb 300 it's the same as the fat cat 6. The wb is chrome instead of black and it comes with the channel hardware for the whisbars, it's also like $40 more. If I had the chance to do it again I'd get the fatcat and buy the hardware. The fatcat/wb 300 is a nice rack though.
Both are longtime reputable brands so take that out of the equation. The Yakima FatCat is the best rooftop ski/snowboard rack currently on the market, PERIOD. I've owned several different styles over the years, and it is definitely the best thus far. Wider than others to accomodate even the widest of boards or as many pairs of skis as you want. Also the most low-pro and aero option out there to minimize MPG loss and no windnoise. Also called the Whispbar WB300. Cargo box is definitely an even better option, although much more costly and more MPG loss.