I'm not an expert, but there are a lot of harmful things added to tobacco and cigarettes during the manufacturing process, that marijuana doesn't have (now who's to say that it might not get processed in a similar way if it was produced as part of a legal, large scale industry).
Things like:
-Formaldehyde
-Arsenic
-Ammonia
-DDT (presumably as a pesticide on the crop?)
Info from this page:
What's In a Cigarette, 599 Ingredients in a Cigarette
Personally, I don't think there's much benefit to legalizing either; but if you give me a choice between pot or tobacco being legal, I'll take pot every time.
