Alcohol is conclusively and irrefutably more dangerous than marijuana over the course of a lifetime or in a single instance. It is virtually impossible to overdoes on marijuana, the stuff just isn't toxic enough and would require more than your bodyweight in nearly pure amounts. The risk of smoking marijuana (the tar and other chemicals created when smoking it) can be reduced to a negligible level depending on the way the marijuana is smoked. It is not particularly addictive. Marijuana has an entirely different effect on brain chemistry than alcohol making it easily more pleasant and a more lucid experience than drunkenness.
Alcohol has a very high toxicity. It is extremely damaging to the liver in the short and long term depending on consumption and dose. It can cause death due to its toxicity in the brain in relatively small doses. Alcohol can cause blackout periods when it reaches toxic levels and, while they may not kill you, you may die by inhaling and choking on your own vomit. Alcohol is known only to be safe in small amounts several times a week. Alcohol is considerably addictive. Alcohol, when a person is drunk, distorts inhibition, lessens reaction time, defuncts reasoning mechanisms in the brain and reduces lucidity. This combination leads people to do dangerous things and can put them at risk of their environment, themselves or others.