I've actually just decided to upgrade my personal stereo. I've been using MD for about 5 years, and whilst I've been pretty happy with the mileage I've got out of the format, I was pretty keen to upgrade to either a HDD or flash-memory based player. In the end, I bought an Hi-MD player - thought I'd outline my reasons, as they may help you arrive at decision.
My reasons were as follows. I wanted something with a reasonably long battery life - I do a couple of long haul flights from London to Australia each year, and listening to music's about the only way to stay sane on board. Having a player's battery go dead half-way through the flight aint an option. Also, I was keen to get something that was capable of running off AA batteries - you can buy them anywhere, carry a couple of spares, and I don't like taking rechargers with me on vacation. I'm not fussed about being able to store my entire collection on the player - 5 albums at a time on a disc is fine with me. And finally, quite a few of the HDD/flash memory players don't support gapless playback - it's a small thing, but hell if my CD player can play Abbey Road the way the Beatles meant it to sound, I expect my personal stereo to do likewise!
Things that put me off buying another MD player - Sony's insistence on Atrac as the only natively supported format, and the bad reputation that SonicStage has. Audio quality wasn't a deciding factor one way or the other - most of the time I listen to my music on trains (and planes), which aren't ideal acoustic environments.
I sat down and worked out the features I needed and wanted in a player, and then compared that against the various players out there. In the end, the Hi-MD players looked pretty good against my criteria, so I went with that. However, none of the factors that were important to me may apply to your situation. If one of the 20GB HDD players does everything you need, go for it!