DEFINITELY spring for discs on a mountain bike. They dissipate heat quicker, and when they get wet, they will also dry quicker. Not to mention, disc brakes just flat-out stop better.
With v-brakes, the gases from the friction and heat built up between the wheel and the brakepad can't escape as well as with disc brakes, and this results in the loss of braking force. Your brake pads overheat and lose their braking capabilities. It's the equivalent of driving your car and making more than a few panic-stops in a row. The first few times the car may stop fairly well, but then all of a sudden, your foot is going to the floor and nothing's happening. While this can happen with disc brakes also, it takes longer than if you have drum brakes.
I also personally find it easier to change a tire on a bicycle with disc brakes.
IMO, the only downside to disc brakes is the chance to bend a rotor, but I've never run into that problem.
