I've considered selling my FoST for a FiST before. It seems if you plan on going BT or really going for high performance, the FiST may be an overall better platform. That being said, since I DD my FoST and drive 350+ miles a week, I don't think I could ever make the change. I'm 5'11 200lbs. and while the FoST's full-leather Recaro's were a little tight when I first got the car, I've gotten used to them and I find them perfect. Because of the FiST's narrower seats, I don't think I would ever be able to get fully used to them and that by itself is pretty much a deal-breaker. Other issues are with the decrease in space...I frequently have friends and family in the back seats and I'd prefer not to get complaints about the lack of room in the back. My job also requires me to frequently carry a lot of equipment to the point where I had my FoST's hatch, back seat, and passenger seat filled.
And then it's just personal preference things...I really don't like the aesthetics of the FiST's rear tail lights and exhaust. I really am not a fan of the FiST's console and dash layout. I don't know, some things are petty and some things are legitimate concerns for me. Ultimately, I don't think the FiST was made for me. Maybe I could make the jump if I was really into performance and tracking but I'm not. I do plan on BT'ing my FoST eventually but as it is, I don't track my car and I have zero plans to do so. So while the FiST may be a better platform, I doubt the difference is really noticeable on the street or on the highway.