obviously you lose some control and other things (being able to look up the melody track etc) of a midi. But you can sync an mp3 with the board sequencer for control functions.

but I think the question is more along the lines of playing along either with a midi file with melody muted or mp3 recorded without melody. Here I don't see that much difference in that with neither are you controlling an arrangement with your left hand, you are playing with a fixed accompaniment. So if you take that as a premise an mp3 of real string or brass orchestra will always sound better than any midi file, even taking into account mp3 compression etc.

The same midi file sounds different on different boards because it depends on the gm sound set. An mp3 sounds the same, and no board will ever sound as good as a recording of a real string orchestra surely?