I think the carcinogenic contents of the smoke are similar. The difference tends to be how much people consume, meaning the dose response. Most people smoke a small number of marijuana cigarettes per month compared to tobacco cigarettes. If somebody were smoking a comparable number of marijuana cigarettes, the risk could be similar, but there aren't that many people who do that.
In terms of second-hand smoke, again, the contents are similar, and that's why governments are moving to ban marijuana smoking wherever smoking is banned.