vees have cable stretch, where as a hydraulic rim brake is very solid. TO answer your question, discs are indeed better in terms of which is better for more conditions. However, vees give a more firm hold, and if you're riding trials i'd doubt you'd be riding in the rain because it reduces the grip your tyres can get. So if you're stuck for choice between disc and vee, get the vee brake - avid single digit 7 or ultimate