First of all sorry for 2 posts above, my internet connection went wappy last night so it must have accepted the post twice.
miden the MP3's I mentioned are what I program for singers and groups that have backing tracks off me. I rarely use these myself other than sometimes when I start off and finish a show. Myself and quite a few others on SZ sequence tracks on their computers and use numerous software synthesisers and packages, so I prefer to record songs as live audio that I do for singers rather than producing the simpler midi files. Also with MP3/Wav I can also record live harmony parts so the song they sing sounds more fuller when played back as it's their own voice doing the harmony rather than an effets machine for example.
I know some midi files can have harmony parts recorded in them but I've not heard how these sound. Fair enough it is midi data that I have played in from a keyboard but I also put real guitar/bass tracks down as I believe no keyboard has come close to the sound of a real guitar yet. Tyros 2 Super articulation and the G70 guitars are getting close though
The pros of MP3/WAV I suppose are not needing a keyboard to play them back on, a simple CD player, mini disc or whatever does fine. They also sound the same every time you play them back. As you mentioned, if you change a keyboard or sound module they will have a different gm/gs set of sounds so midi files that may sound great on a Yamaha for example would sound completely different on perhaps a Ketron until you tweak the sounds used. This is obviously boring at times and time consuming having to go through a large set list of songs every time you do an upgrade. I like to record just enough that still leaves the melody for me to play live as there's no way I'd fake anything. Songs such as the likes of Queen, in order to sound as close to the original as possible just have too much going off in them, so some backing is required to pay them justice.
Downfalls of MP3 are that once recorded you are stuck with the sounds and key they are recorded in. You can use software to transpose MP3 (timestretch etc) which does work and keeps the song at the same tempo, but to change sounds you need the master data loaded back into the computer. If you have guitar parts recorded then you decide to get a guitarist for your band, you've had it as all the songs will need to be redone with the guitars missed out.
With midi files a simple press of a button changes the tempo, the sounds used, the volumes of each track, the key its recorded in.... so they do have their uses in that respect.
My personal taste is that I think real live instruments recorded as a MP3 song will always sound better than a keyboards playback of a midi file but it's each to their own. Whatever song I play live I like to pick out all of the instruments being used on the original and then I play it back as near to the original as possible, but I put my own personal stamp onto it
Check out an old friend of mines site. He sells midi and audio files which are of a high quality. Play the demo of the midi file you want and then play the audio demo (without singing if available) you can usually tell that the audio has far more going for it.
Here's his link
http://www.songgalaxy.com/