I have Silk 850 and 1050 and had Sk8 950.
I agree with foilstate description :
SK8 is faster, more glide, pumps better, equal foot pressure at top speed, the Silk surfs better, more stable and solid in the turbulent water, front foot pressure at top speed.
Just one thing about pumpability and glide on the Silk ; I would not say it is harder, but it needs to be fine tuned whereas the Sk8 is much more plug and play : just put it somewhere in the track and it will work fine. Once you get the correct settings, the Silk glide and pump nicely.