After using am 8" disc on my jump bike with contaminated pads and a bad bleed I've found its stopping force is quite sufficient for trials. Back when it was new it was amazingly good at biting and holding. It does seem slightly slower though, so for pure response time alone a rim brake usually wins as its a fairly important factor in trials. The disc is quieter which is nice and it works very well in the rain. That aside i never usually ride in the rain so its somewhat of a non issue. After pad replacements, levers, hoses and what not the disc brake has worked out a lot cheaper so if your using it for a long time its a sound investment. I reckon i'd be quite happy using a rear disc on a trials bike if i was into comps every sunday in any weather on some really varied and unyielding terrain, for a street ride every now and again though a rim brake is my choice due to a faster full lock time and more locked feel whilst on the rear wheel (spoke flex with a disc feels "wobbly").