I don't have a Ferris dealer, so I can only browse them on the net. However, the 600Z has a true independent front suspension, vs the 400S only having a pivoting axle. It also has ZT-3200 hydros vs the ZT-2800's on the 400. To me, those two things alone would be worth the extra $800. The 600Z also has a Kawasaki FS600V option. I'm not familiar with Briggs Commercial, so I can't say whether it's a better option over the Kawi FR or FS series. From the pictures, the only thing that stands out to me is that there are no anti-scalping wheels in the center of the deck. It might not be an issue, but pretty much all the other brands have them.