Auto Input Monitoring will allow you to monitor the live input to a track when in record or idle mode, but in playback mode, it will always play the audio which is on the track (or be silent if there is no recorded audio), even if record enable is on.

Input Monitor Only will always monitor the live input to a track, regardless if the track is in idle, record, or play mode. Therefore, if you have a track in record enable mode, it will not playback what's recorded on the track without taking it out of record enable.