I think alcohol is better for the simple reason that the negative consequences are more immediately obvious. Cannabis is much more sinister in that the negatives usually creep up on you before you notice and so long as it has you in its grip you may not even notice until you stop.

I've witnessed people have acute psychotic breaks after smoking strong cannabis for the first time, and met one man with schizophrenia who attributed his condition to his past cannabis use. Whether he's right I don't know, but the connection is at least well acknowledged nowadays.