One big difference, though, is that weed itself is only very mildly physically addictive, and in a small percentage of the population. This is as compared with tobacco, which is more addictive than heroin, for pretty much everyone.
Also, weed contains chemicals, like THC, which are not so harmful, and can be ingested in ways other than smoking (e.g. inhalers). And unlike cigarettes it has benefits as a painkiller for people for whom other painkillers no longer work (e.g. terminal cancer patients, who probably don't need to worry about getting lung cancer 20 years from now).
Overall the scorecard seems to be:
Smoking weed is probably just as bad as smoking cigarettes, but has a number of benefits over cigarettes and is nowhere near as addictive. That makes it less harmful overall.