In the papers, studies and news stories that I've read, marijuana smoke was indeed found to contain more tar and carcinogens than cigarette smoke.
that statement is still widely misleading tho... in comparison people smoke way less joints in a day then cigarettes, well for the most part anyways, for me i smoke a pack a day, and i don't smoke 5 joints a day, so i'm getting less ammounts of tar and carcinogens from my weed smoking then my cigarettes... i'm sure this rings true for mostly everyone as well...