Mistered 4111,
It seems you already have rather excellent
equipment. I would recommend you use
your 9000 to play midi files, and only if
that doesn't suite your needs as far as
sound and convenience, go to a laptop.
Whether you go with the 9000 or laptop, I would also suggest to transfer all your
Midi files to audio CD-Rs, and use a CD
player as your backup.(CD-RWs would be even better, if you can find a CD player that will play CD-RWs).

Another option for backup is the Nomad MP3 player/recorder which has an internal hard drive.

I have used midi files converted to
audio CD-Rs with Cakewalk using the Roland
VSC DXi plugin, and have had excellent
results.(My CD player unfortunately doesn't play CD-RWs).
I agree with the others, forget MD.
Hopefully,very soon the price of portable DVD players with display, will drop enough to replace CD players. Imagine all the backup .wav
or .mp3 files you could put on a DVD-R/RW.
I hope portable DVD manufacturers make something affordable and practical with the musician in mind(such as remote control, and
changing keys without changing speed, etc.).