I rode a Trek Soho and I hated the acceleration feel due to it being belt driven. Acceleration from a stop was mushy and no where near as crisp when driven by chain. Granted that the Soho is not a fixed gear I would only imagine it would be worse for fixies to be belt driven as you'll be also using the drivetrain as your brakes. So IMHO, do NOT go with the belt. Stick with the standard chain.