In rugby I myself had experienced concussions from collisions at least once a season. Anyone I know who has played have experienced similar.
Less protection to the head means lower g-force hits sure, but still I've seen players knocked unconscious by pure body collision hits or an accidental knee to the head (this happens a lot more than you think) ... in fact in my 4 years of high school football I rarely, if ever, saw anyone knocked unconscious... where in rugby I can recall a handful of incidents.
Rugby has no room to outshine football in this manner. Such a foolish arguement.