i think the minidisc was meant to be an alternative to the CD, a format that could be easily written and re-written, that was very compact, and much more portable and less cumbersome than CDs.

whereas the memory stick seems to be more of an attempt by sony to make more money, and dominate another part of the electronics market. they'd rather have people that buy sony products ALSO buy their solid state memory cards, instead of being able to shop around and buy 3rd party flash memory from random taiwanese companies that sell it for much less. there's no obvious advantage of memory sticks, they're just kind of cool.

minidiscs didn't catch on in america because sony did a shitty job of promoting it, and the equipment was way too expensive until recently. they started out with a couple really expensive portable players that were too big and heavy at a time where the cassette walkman was still prevalent. they compounded this by attempting to offer a small selection of artists on the sony label on minidisc, and not at all locations. they've basically lost their chance now, because of portable MP3 players. however, consider this...
a minidisc holds 100 mb of information. its a proprietary audio format, but exactly a CDs worth (74 or 80 minutes).

a five pack of 80 minute MDs costs about $10 - $12.
the price of a 16 mb compact flash chip (which will hold maybe four or five mp3s) is about $14, $15.

flash memory is still a huge amount more expensive than minidiscs are. true, flash chips can be written to very easily. but with newer MD units with things like NetMD, you can transfer audio to minidiscs from computers much faster than playback speed - up to 64x on certain models. recent upgrades in the software side of minidisc technology has made it possible to write 160 minutes of stereo audio to a single, ordinary minidisc at a higher quality (132 Kbps bitrate) than the somewhat-standard MP3 quality of 128 Kbps. up the MP3 quality and you'll only be able to hold less and less on one piece of memory.

anyways, theres a good reason why minidisc is still very popular in asia an europe. just because its not popular in america doesn't make it suck