The main difference I see is from advertisments. The people who smoke the light are "Alive with Pleasure" and the ones who smoke regular are in "Flavour Country". ;-)
Seriously though, I think you're right about "light" cigarettes being more harmful. I've also heard that cigarettes are more harmful than cigars because of carcinogens in the cigarette paper and chemicals used in processing.
It would be even less harmful if you didn't smoke at all.
