I prefer calipers over disc.
However, when the time comes, I plan on using the "it would be nice (and safer) to have a bike with disc brakes" excuse to the wife when I want a new ride.
But seriously, it is pretty hilly in my location. I tend to down-hill pretty aggressively, and there has been a time or 2 when I thought disc brakes might give me a bit more grabbing power. Sure, I could adjust my riding style, too.