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.
The OGG format is an open and free standard with no patent and license restrictions. Nowadays, there is no clear standard for this format. Also you should understand that OGG is just a container for video and sound, and may contain streams encoded by different audio and video codecs.