You shouldn't be taking any medication over the counter for a very long time. But Tylenol PM is typically used for when your sick and trying to sleep. It contains an Anti-Tussive (which is really just fancy talk for Anti-Cough), Fever Reducer, and also contains an Anti-Histamine as well. If your not coughing, having an allergic reaction, or suffering from a fever than you dont necessarily have to be taking Tylenol PM just for sleep. Another thing is that Tylenol PM doesn't contain Dextromorphans. DXM as they call it, is the chemical in many of the other OTC medications such as Robitussin and Coracidin. DXM is addictive and produces a hallucinogenic effect but I haven't heard of it being in Tylenol PM
Ambien or it's generic Zolpidem Tartrate is a prescription drug that is used for sleeping disorders. If your having trouble sleeping and if it's a medical necessity, than go see your doctor about a sleep medication.