On a mountain bike? Oh, this is going to be easy.

A mountain bike with disc brakes has the disc higher off the ground than the rim (the braking surface of rim brakes). This means that less mud gets kicked up onto the braking surface, ensuring that you can brake more sharply. This also aids modulation, making you more sure of your brakes.

Negatives? Disc brakes are slightly heavier than rim brakes… but this is a mountain bike.

Disc brakes are unequivocally the best braking system for mountain bikes.