The comments above pretty much explain the difference.
I find that using MIDI files to create a backing track is better than an mp3 because of the editing capabilities, especially when transposing to another key is necessary ... If you transpose an mp3 more than 2 or 3 steps - especially if it has background or harmony vocals in it - you are really going to distort the sound of the vocals ... I will sometimes take a MIDI file, change the instrument voices, drum set, key, tempo, whatever, and then record background vocals through the keyboard ... this produces an mp3 that sounds as I want it to ... OR record the MIDI file to a digital recorder and add more than one vocal track ...

Of course one can always just edit the styles on the KB to create a backing track as well, but that was not the question ...