what's the difference?

[waitee]

what are the main differences between an MP3 player & a MINI DISC PLAYER?

[jenky]

Mainly, you'll have to physically insert a disc containing an album or so into a mini-disc player, whereas you download tracks from your computer on to an MP3 player. The volume of music you can fit on an MP3 player depends on the size of it's memory.

[Mattk]

an mp3 player is solid state(no moving parts) whereas a mini-disc player needs loading and has moving parts

[nicola_red]

Convenience, in that with an mp3 player you don't have to cart discs around with you and fish them out of your bag when you want to listen to a particular thing. And for that privilege you will pay extra!

[Greedyfly]

I have an mp3/minidisc player which is both. You use the software to download your tracks then put them onto a disc for the player. If I were to advise on which to buy I would say mp3 player wins hands down. Easy to use, generally small and great sound.

[Jeanalesi]

Also with a mini-disc player you don't always need a PC to copy you can plug it straight into a CD player or stereo, which I was under the impression you couldn't do with an MP3 player

[Mattk]

also mp3 player jukeboxes(ipod etc) have huge storage capacities capable of holding your entire record collection !

[Dazzler]

Up until the recent price drop in memory cards, I would have said MiniDisc for the flexibilty of carrying a larger number of tracks easily, but now with media so cheap you can afford to have multiple cards, as long as you player supports them. I don't have experience of the hard-drive players, but they seem to have huge capacities, although battery life would almost certainly be shorter than a solid-state player. All the other answers are valid too, just adding a few thoughts, have fun.