FM has 15kHz of bandwidth per stereo channel or an effective 30kHz sampling rate. The rest of the space is used for supplemental signals, including, the "pilot carrier" that is used to generate the "stereo image." There is space for three more full bandwidth mono channels on the end of an FM broadcast. One of them is often used for RBDS.

FM signals receive AM interference but heterodynes exclude them effectively. The cost is vulnerability to multipath reception in highly signal reflective environments and capture/wandering effects when two signals of similar strength are present.

AM _can_ sound pretty good. Most AM transmitter sites are poorly maintained, combined with other stations into one antenna system (something you can do on AM with a phasor), and are typically just simulcasts of FM content or satellite delivered content. There's no real care put into it. On a well maintained, tuned, and properly programmed station, mono content on AM sounds quite pleasant.

That's not even getting into "cost saving" measures that AM operators employ that completely compromise their signals. Or what Nielsen has convinced them to inject into their signals to register modern "ratings points" from the "portable people meter" system.

Guess where I used to work.