Cannabis itself is non-toxic and cannot be overdosed on - it is physically impossible to smoke enough in a short enough period of time. There is no known medical fatality purely resulting from cannabis use. Tobacco has long been known to have high tar content and numerous carcinogenic (cancer causing) chemical chemicals. Smoking tobacco regularly increases the risk of lung cancer by a fair amount. The medical research and evidence against tobacco is clear and unquestionable - tobacco is toxic and smoking heavily is unhealthy. If by toxic, one refers to the amount of damage each drug does, then there is much argument to the claim that tobacco is more damaging than cannabis.
Whilst there have been many claims that cannabis is the equivalent of smoking any such number of cigarettes have been misinformed, there is not firm evidence, rather unrealistic assumptions of the experiments. Smoking anything is bad for you. Regarding cannabis there is much evidence on either side. There is very little evidence supporting a link between cannabis smoking and bad health - there is practically no difference in the levels of lung cancer between non smokers and cannabis smokers.
One only has to look at the body of evidence over cannabis' medicinal qualites to decide which is more harmful. In a certain study, the main active psychoactive compound, THC, was tested to reduce particular kinds of tumours as well as inhibition of certrain cancer causing chemicals in the body.
The levels of tar in low quality cannabis is higher than in decent quality cannabis as it includes stalk and leaf which possess higher tar content. Cannabis does however not have numerous cancer-causing chemicals added, unlike cigarettes, although prohibition does reduce the quality as the cannabis market is in the criminal hands of the black market. Some research has shown cannabis to be useful as a "bronchial dilator." And it is the high quality buds of the flowering tops, which have a higher proportion of THC than the rest of the plant, and so produce less tar.
In terms of toxicity, tobacco is highly toxic. Cannabis is non-toxic, and is actually one of the safest thereupeutic drugs known to man in the entire medical field. Taking over 150mg/kg of aspirin can have serious and even deadly results if untreated. For a small adult, thats almost equal to ingestingg 20 tablets containing 325mg aspirin and lower levels can affect children. Tobacco is far more toxic than cannabis in medical terms. The vast majority of evidence suggests that smoking tobacco regularly is more dangerous to your health than smoking cannabis regularly.