Disc brakes are awesome as far as braking power and smoothness are concerned, but they are expensive, heavy and a hassle to maintain. I would get regular old-fashioned rim brakes. But I also wouldn't go down that hill at 40 mph when it's raining, disc brakes or not (for one thing, surfaces are much more slippery in the rain, so I'd be worrying about losing control...). If you insist on those speeds in the rain, disc brakes might have a real safety advantage.