Both offer their pros and cons.
Many Trials riders prefer to use a rim brake on the rear, because generally they offer better hold if set up well and a disc brake up front for more modulation, to help with things like stoppies and front wheel moves etc. Having said that though there are many mod riders nowadays that go down the dual disc route. Discs can be pretty damn powerful on mods but not so much on stock, because of the laws of physics. (bigger wheels)
Discs can be hard to maintain as well and are a lot easier to break than a rim brake because of the rotor being vulnerable to hitting. With rim brakes, they are generally more tucked away. If anything should go wrong, the rim brake is generally cheaper to repair than a disc also. Also, if you have some big ass rotors on there, they can weigh a lot more that the rim brakes.
EDIT: Yer, pretty much what the guy above me said.