MPG does not play in browsers or on phones and is far larger than MP4. M4A sounds better than MP3 at the same file size and is native to Apple devices. MPG is at home in DVDs, Video CDs and old camcorder footage; M4A is the usual choice for music and voice memos on Apple devices. The M4A contains the soundtrack only; the picture is dropped.
The defaults suit most files. Under Options you can choose the bitrate and trim the start and end. This is demanding work for a browser: a long or high-resolution file can take several minutes, and your device does all of it, so keep the tab open.
This pair uses the compatibility engine, ffmpeg built for the browser, because MPG is not handled by the browser's built-in codecs. It is a one-off download of about 10 MB that the browser keeps, it is slower than the built-in codecs, and it takes files up to 1 GB. M4A is written at 192 kbps unless you choose another bitrate.