Not all belts are created equal, but I do like the shaft mainly because its sealed and reliable (for me). I had a bad experience with the XX4 where 3 belts snapped - im told because I had badly adjusted the slipper.
I badly adjusted the slipper on my BJ4 at the Euros - and it wheelied !!!!!! give me wheelies over snapping belts !
Seriously though, it depends what you like. I think the belt does give a little smoother power delivery