I saw this news thing once, where they tested a guys brake time reactions on weed and on booze (on different days, obviously). He was way better on weed, and he was a lot more alert and cautious.

Saying that, you shouldn't operate heavy machinery when you're under the influence of either, so it's really a moot point. You're prone to doing stupid things when you're on both alcohol and THC, but you're less likely to get off the couch and beat the shit out of your wife when you've had one too many bongs.