In broadcasting, FM (frequency modulation) is much more relevant, as it describes a type of radio signal. PM (post meridian) describes time of day. Although, in general, broadcast advertisers prefer PM times over AM.
If you meant to ask about FM vs. AM radio signals, FM is preferable because:
- it offers higher-fidelity sound
- it allows for stereo signals
- it requires higher bandwidth than AM (which makes FM more expensive to produce), and therefore offers better quality sound by reducing interference and background noise
- it offers reduced signal fading
- modern tuners have the ability to differentiate between 2 or more signals sharing the same channel and filter out all but the strongest
- station numbers are shorter and easier to remember
