Spartacus, If it were me in your situation, I'd just get the FXS, and ride it as is on dirt.
For casual trail riding and fire roads, it'll be a bit loose, but otherwise should be fine. I'd just be a bit worried about denting and/or cracking the rims on square edges(like rocks), but they should be ok if you're careful. I'd run a fairly high pressure(at least 18, although not ideal for grip) in the tyres to prevent bottoming on the rim. I've been riding my FX on dirt with the stock tyres and pressures, 'cause it has to do road aswell. I don't push my FX too hard off road, I mainly use it for some adventurous casual riding by myself, because I can go nearly anywhere on it, due to it's queitness. I've even ridden sand/hard-pack on road tyres before, on a KLX250 I used to have; it's not ideal, but possible, and ok for non competitive fun. If you are riding with mates on knobbies, they will definitely have the advantage over you, though.
Edit: oops, a lot of what I said has been said already, 'cause I replied to the first page, and didn't see the second page!