Well, for one thing building this bike up with discs will cost a LOT more money since I'd essentially need a completely new set of rims. Also, there have been numerous articles out on the web regarding cantilever vs. V-brakes vs. disc brakes, and while disc brakes get a lot of "pros" for stopping power, they have issues with there being so many types that you're likely to run into difficulty getting replacement parts at shops because they may not have the particular brand of disc brakes you are using. Anyway, you can always find replacement pads for pretty much any type of cantilever or V-brake. Anyway, I'm using my old mountain bike rims, so that means rim brakes.