To be honest, beyond the obvious fall in weed related arrests I wouldn't expect legalisation of the stuff to have much of an effect on crime either way above the background noise level, at least in the short term. You might be able to tease something out with a long term epidemiological study, but that will take a lot of time and a lot of data points. I could be wrong but I suspect that reality will be stubbornly a lot more boring on this particular topic than either the doomsayers or the evangelists will like. On a long term, I could see some real and lasting benefits from the reduced arrests and incarcerations and the social identity effects of dissociating the use of the drug from crime.