I love this thread ... Quick question for the OP, do you have the means to test ride the bikes you are considering ? ... or do you base all your decisions on on line reviews ?
PS: I sometimes get caught in the rain & noticed that gray stuff that ends up all over my bike... I'm sure if it had a belt, it too would get dirty...
PSS: Check the fat bike forums, some guys are having bad luck with their belts ( breaking )...usually from sand building up between the cog and belt. ( one member even tried to make a "chain guard" to keep the sand from the rear tire, falling right on the belt. )
Good luck with your search .