OK all you engineers! Let's say it takes the same amount of energy dissipation to bring a bike from rolling to a stop whether the brakes are rim or disc. If the bike has rim brakes and the rim slows then the spokes have to transmit the stopping force to the hub. Yes? If the bike has disc brakes and the hub slows then the spokes have to transmit the stopping force to the rim. (yes?)
Either system will generate about the same amount of heat as either system is absorbing about the same amount of energy. In the rim brake the heat goes into the rim. In the disc brake it goes into the disc. Movement through the air dissipates the heat.
As a practical matter we have ridden rim, rim with drum, and disc brakes in mountainous terrain. All were properly adjusted and all stopped our bike safely. (320 lb team) We prefer the discs because:
1. The rim does not get worn down.
2. The pads on the disc have a very short distance of travel which means that the large motion of the brake levers allows much better modulation of the brakes.
3. Almost everyone that I have heard say they would never use discs--have never tried them.
All this aside I think that whichever system is used the best pads should be fitted, the system properly adjusted, and the wheel should be built with quality spokes and be strong. This may be more important than which system is used.