M4R or iPhone Ringtone is the format of ringtones for mobile devices from Apple. This is the same format as M4A but with it has several limitations. The duration of the audio stream should not exceed 40 seconds. In case the duration is longer, iTunes does not install the ringtone on the mobile device.
Mp3 is the most popular and widespread audio format. It was created by MPEG team in the 90s of the last century. MP3 format provides the quality of sound reproduction, similar to the original sound. The MP3 format is based on a lossless compression algorithm made to reduce the data size. In other words, music streams are compressed and deleted in the format that are somehow similar for listening on devices of common use.