They both put tar in your lungs. A tobacco smoker generally smokes far more cigarettes than a cannabis smoker. Per-breath, cannabis seems to put more tar in your lungs, since it's generally unfiltered. But a tobacco smoker takes many more breaths of smoke.

Some information here:

https://www.drugabuse.gov/publications/research-reports/mari...

As far as I can tell, vaping is far, far safer than inhaling smoke, for both tobacco and cannabis. The healthiest strategy is to avoid all of these things of course.