From what i've heard, the Stype's good for slalom and freeride. The stype I saw had the cams starting on the second batten which I certainly don't like as I like the sail to be 'baggy'on the bottom. For formula they are OK and If you are looking for light wind the 8.5 is good. Do they have an 8.5? I think they don't have that number.
Good luck searching for your sail,