Me - I would rather see them attack it from both directions. Decriminalize marijuana at the federal level - and then let the states decide. Some would probably react by increasing criminal penalties for marijuana. Some would legalize it. But it would be 10th amendment issue.
Then, reallocate the police resources currently consumed with marijuana arrests (paperwork, court time, jail population costs, etc.) and roll them on to heroin, cocaine, crack, and meth. The user population is much smaller for those drugs, and the increased police activity and increased money available should make a serious dent in the crime in that area. Also, at the federal level - reallocate cops and dollars that currently focus on marijuana to focus on the illegal immigrant problem.
That's how I would handle it.