Conditional discharge, often referred to as "drug first offender." The reason is that conditional discharge is narrower than first offender.
To be eligible for a first offender plea, you must (1) have never been convicted of a felony (2) not already used your First Offender plea, and (3) not be charged with DUI, a serious violent felony, serious sexual offense, a charge related to child pornography or electronic sexual exploitation of a minor, or a serious crime committed against a law enforcement officer engaged in his or her duties.
To be eligible for conditional discharge, you must (1) have never been convicted of a felony (2) not already used your conditional discharge, and (3) be charged with possessing a narcotic, stimulant, depressant, or hallucinogenic drug, or marijuana, or be charged with a nonviolent property crime that is related to your addition to a controlled substance or alcohol.
So if you're charged with a drug offense, it's better to use conditional discharge because it saves your First Offender plea if you're later charged for any crime. But if you use First Offender, then your conditional discharge can only be used for a drug crime later.