Interesting debate - I'm a really big MD fan (I luv my Sony MZ-R55 even though it's an old model now) - but use MP3 extensively at work (have about 1000 that make up my playlist on my PC).
I'm not sure of the exact data size details but don't forget to bear in mind that MP3 can be encoded at different bitrates - 128k ios standard adn I think acceptable (but nowhere near a digital CD to MD copy quality)... but you can go as far as 320k where the quality is superb and file sizes are HUGE!
Also, Sony are making MD models now that can, via a USB connection, rip MP3 onto a MD which is nice and versatile! Not sure whether you can mix MP3 and audio MD on one disc though...?
I've never tried using a portable MP3 player - mainly because (exclusing the Sony 'Network' models) they are all so ugly
Hersh