Cigarette per cigarette, marijuana is hugely more toxic than tobacco - studies suggest 1 marijuana cigarette is roughly equivalent to 10-30 tobacco cigarettes. Marijuana is strongly linked to a number of lung diseases such as lung cancer, including an otherwise rare, severe destructive emphysema.
Marijuana is also known to precipitate mental illness, such as schizophrenia. It is also associated with severe anxiety or panic attacks immediately after use.
Heavy long-term use is strongly linked to loss of mental function, especially short-term memory. In the most severe cases, abusers have been left with essentially no useful memory function.
Other risks involve impairment of skilled tasks such as driving. Tests involving subjects using a driving simulator have shown that 1 marijuana cigarette can impair driving to the same extent as the legal limit of alcohol for up to 36 hours.
The effects of cigarette smoking on health are well known:
Lung cancer is the most common cancer in Western countries, yet in non-smokers it is not common. Smoking even 10 cigarettes per day increases your risk of lung cancer by 10x. Smoking 40/day increases risk by almost 50x.
Smoking is also responsible for the vast majority of cases of chronic bronchitis or emphysema - which result in enormous disability.
Heart disease, and other arterial disease is also linked to cigarette smoking.
Cigarettes are also tremendously addictive - some have estimated them to be more addictive than heroin, and approaching cocaine.
In short, they're both bad, and I wouldn't smoke either.