Ethics -- You state you already have a public defender as your counsel. That ethically prevents any reputable attorney on Avvo - or anywhere else for that matter - from weighing in. Professional ethics prohibits interference with an existing established attorney client relationship.
This site is not here to provide a forum to second guess your attorney.
Discuss this with your attorney.
I fully agree with the first answer given. You must rely upon your PD and go with your own gut as to what's right for you. However, I can tell you that if you had a dirty urine, it does constitute a Violation of Probation and that means you don't have a lot of latitude anyway as to what the plea will be for you. Getting your drug counselor involved is usually a good idea.