yes, apparently rollerbrake is just a glorified drum brake..
http://www.sheldonbrown.com/gloss_ri-z.html
the weight for the two hubs, once you add the disc and caliper to the Alfine, is about the same, a little shy of 2kg....
you are right in thinking that the drum brake is more protected from potential bashing when folding the bike, on the other hand a disc brake is essentially mounted on the axle so it does stop the small wheels well, and is less prone to fading on long descents (which when touring in the mountains with gear is IMPORTANT!)
all in all, I am happy to stick with the Alfine....