Disc brakes stop better and more consistently... period.

If you performed a blind study where 32mm wide tires at 85psi were used on two different frames - one super lightweight and the other with a beefed up rear end to accomodate disc brakes, all other things being equal - nobody would notice the difference in frame stiffness, but most would llikely notice the difference in stopping power. THere is less and less reason to use rim brakes on any bike. I think there are some racing categories where discs are not yet fully accepted, but otherwise discs are preferable in just about any situation. I think they might be a couple ounces heavier than high-end rim brakes.

Also, the flex in a super lightweight steel frame can make it less useful or comfortable for hauling stuff, although you specifically said you would only ever use 'light' panniers... you never know when the mood will strike to run to the grocery or hardware store to take advantage of a sale on potatoes or cat litter or sheep manure - you will appreciate a nice stiff frame.