One time thing: pot, since they are more likely to colapse on a couch and chuckle all right at random shit. As opposed to falling down, running into shit, and puking off booze.
If it is a continuously: neither; if that's not a choice, as much as it hurts to say, since i despise the stuff, pot.
I have known alcoholics and stoners, hell i was an alcoholic(or is it: i am one, just dry??). I have seen both substances destroy lives. So, i'd prefer neither. Pot by itself can be rather harmless, provided it's straight weed, not mixed w/ other shit(dust, blow, crack, etc.). Unfortunately, it can lead to other drugs.