These discussions are always interesting.
The way I see it (if you ignore the effect of the type of person on the effects) is all drugs, foods, drinks, anything really you can consume will be harmful at the wrong intake levels. What makes drugs particularly significant is that at small levels (e.g. milligrams), they can show significant effects. Comparing nicotine to THC quantitatively is quite difficult, as the argument isn't exactly scientific. They both cause harm. One is legal, due almost entirely to it's history and very little to do with the harm it can cause. Most of the side-effects of both drug use is from impurities in the form they are obtained, not the effects of the chemicals them self. Either drug used in moderation would be most likely sustainable and have minimal effect on longevity - as with alcohol.
The problem most of the time is that moderation is not practically feasible. The illegality of THC in most countries prevents accurate regulation of concentration and impurities. The social bias towards the cigarrete rather than nicotine patches/injections ensures that the impurities are always present. From this I'm trying to say - most of the harm that occurs from THC and nicotine use has little to do with the drugs, but to do with society and the barriers to moderation as well as the influence of impurities. Alcohol suffers less from these issues because of it's history. All are potentially bad, and useful in specific physiological states at appropriate quantities.