I would recommend more the Midi files, since you can transpose the song to a suitable or easier key for you.
Since I came also from an Organ background, I did use (LONG TIME ago) the Yamaha EL 90 and the module Roland RA95 ( Then both NEW !!)...and worked wonderfully.
The MP3s sound good, BUT no transpose or even the ability to MUTE perhaps some to busy instrumentation.
In the Midi File you can do all that, and also if it is a fast song, you can slow it down to practice until you gain speed/tempo without changing the pitch or key.
I recommend you HIT TRAKS. They also sell MP3's.

Here is the link: http://www.hittrax.com.au/advsearch_H.asp?Dealer=1016&InetOrder=True

I hope it helps you.

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mdorantes