You don't specifically need either, it all depends on the performance you want. Both hydraulic rim and hydraulic disc offer great power and hold. Even good V's are more than capable for trials. It all depends how much you'll have to spend. If you can afford a front disc why not? the only way you can run duel disc is if you run a disc compatible frame. The advantages to a disc as i see it are more modulation, better stopping power. Disadvantages, chance to bend the rotor, some can be a right bitch to maintain. Thats only my opinion, if i were to ride trials again, i would go hydraulic rim front and rear. I just had too much trouble with my disc, it wasn't worth the effort. Its all up to you, take my advice and juggle it around in your noggin' and come up with a decision yourself.