Of the two, the rim brakes work far better and adjust more correctly more easily than the disc, which often can't even stay adjusted to not rub the rotor and whine for more than a few minutes of riding and a couple of stops, from 15-20MPH (stopping a 300lbs-ish total mass).
The disc brakes work pretty well for a stop or two, or at least did when I first installed them, but even after very low mileage on them they began to slip, squeal, whine, not grab well, etc. No amount of adjusting them fixes them for more than a couple of stops at best, and sometimes not even for a single complete stop.
The rim brakes also have far better sensitivity and range of braking than the disc, which are almost on/off once adjustment fails.
Stopping hard (enough to skid the wheel normally) will not even lockup the wheel with the disc now, though I can do that with the rim brakes if I adjust them close enough--I don't like them adjusted to be able to skid the wheel, though, as that's essentially loss of steering control once you do, so I adjust them so that even a panic squeeze can't lockup the front wheel. (though the bike law here requires you to have brakes that can cause the wheels to skid on dry pavement; actually having them adjusted that way doesnt' usally help you stop safely, so I don't).
I have also used the same rim brakes on a crappy noname steel fork, which flexed a lot and gave very poor braking (allowed lowers to twist at the bosses, I guess?). Didnt' try the disc on them as there was no caliper mount and rotor wouldn't've cleared the lowers anyway.
So...as long as your fork is made well enough, decent rim brakes will probably outperform crappy disc brakes by a lot.
