Altho Minidisc is assumed to be at the bottom of the sound quality chain, I have heard it used as backup sounding excellent. Ed, with your PA gear, whichever format you use, if done properly, will sound very good.

I would choose one that allows you to incorporate audio as well as midi. That would be MD, CD, or Mp3 from either your laptop or one of the newer Mp3 players. There are a few forum members using these players. On the other hand, UD and Donny will be quick to point out that nothing beats the convenience and ease of use of laptop mp3s.