you can't simply reduce the issue down to "which one is better, rim or disc?"

a good rim brake is better than a crappy disc brake and a good disc brake is better than a crappy rim brake (duh).

as for generalities, in dry and clean riding conditions, i'd be hard pressed to tell you that a good set of properly adjusted rim or disc brakes are noticeably better than the other. however, in wet and/or messy riding conditions (like winter commuting in chicago for me), i've found the disc brakes on my current winter bike to be superior to the rim brakes i had on my old winter bike. i also like the fact that when i'm commuting in sloppy winter conditions, my rims aren't being attacked by all of the grime and grit from the road, rather the rotor takes that abuse (rotors are relatively inexpensive). and the rotor, by virtue of its placement closer to the hub, simply picks up A LOT less grime and grit from the road than a wheel rim does.