FSI owner here, had a “team” last year and a “black Inc” this year. Brilliant bikes. Extremely light and great to ride.
The made difference between the older F29 and the FSI is the geometry, The FSI has a slacker head angle and is a little shorter. This means the bikes climbs like a race bike but its when you start descending it really comes into its own! Its very nimble and deals with steep stuff very well with out leaving you feeling like your going to go over the bars.
The FSI is also has Cannondale’s newest lefty fork, its a fantastic fork and when working feels like the best fork I have ever had the opportunity to use. However does take some regular maintenance with bearing re-sets here and there.
Over all if your after a light XC bike that’s fast yet still a hoot to ride on the rough stuff then the FSI is the machine!