The MOR (morphine) urine test detects the consumption of any opium derivative, including heroin. The 6MAM test, on the other hand, only detects heroin consumption, but not other opiates.
For monitoring a person undergoing substitution treatment (Methadone®, Subutex®), you should use the MOR urine test. This is because a heroin user who relapses will not necessarily consume heroin, but may consume any opium derivative, such as morphine or codeine. Vigilance therefore requires broad-spectrum screening.
The 6MAM test should be preferred when it is necessary to know whether the opiate consumed is specifically heroin.