Weed's fine. If you have a history of mental disorders in your family or you're already screwed up, stay away from it. Everyone else? Have fun. Tobacco is only legal because governments tax it and make major money from it, I'm sure if they could make significant money from weed, it would legal too. However, they've already dug themselves a huge hole of "LOL WEED MAKES YOU MAD AND FORCES YOU TO TURN TO HARDER DRUGS AND KILL PEOPLE" propaganda that if they suddenly legalised + taxed it, it would be obvious that they're doing it for money.
There are no longterm harmful physiological effects from consumption of the substance (aside from the effects of smoke inhalation, but then again almost everything is harmful when you combust it, so "LOL IT CAUSES LUNG CANCER" is a terrible argument). There's no doubt that it can cause mental illness in people who are predisposed to mental illness, but so does stress (in addition to giving you hypertension), and smoking a joint is pretty stress-relieving if you ask me.
Tobacco on the other hand, is harmful however you consume it, and is massively addictive. I've smoked a bunch of cigarettes in the past and I don't feel any addiction or attraction to it, but some people are hopelessly addicted. Imagine if they made tobacco illegal, all of those addicts would become like major heroin addicts, it's that bad. Not to mention that it's brutal on your body as a whole. Nicotine was used as a bloody insecticide, and there's plenty of other disgusting substances in there (not as many as the anti-smoking propaganda would have you believe though).
There's also the fact that when you criminalise something, you create criminals out of otherwise "good" people.