i think when it comes down to it, alcohol has the potential to harm your body much more drastically and in a much shorter period of time.
considering the addictive properties of alcohol, it is more likely to be abused and thereby cause a more rapid deterioration of the body's internal organs.
i think that people who smoke marijuana, by and large smoke less frequently than a drinker would drink. and hey - it's not addictive!
also, consider the potential for immediate bodily harm. to my knowledge s**t like car crashes, fights, and rape tend to happen when people drink too much. (the worst that ever happens when i smoke too much is i fall asleep while watching comedy central and wake up 2 hours later with my hand inside a chip bag.)
everything in moderation, for sure, but alcohol will definitely hurt your body faster and to a much greater degree.