it really depends on your need.
i'm looking for a portable player too and here's my current overview of the situation
HD Based Players ->
+large storage capacity
+support for different codecs (again depends on the specific hd player, all supports mp3 and some support codecs like flac, wma etc.)
+depending on model, shuffling technology (ipod plays most fav tunes, rio karma has its own algorithm)
-battery life
for all HD players, the battery runs out before you can even listen to all the songs you would want to listen to on the player. its like buying more milk than you can drink before it expires.. in a way. then again, you won't run out of milk since there's plenty
-internal batteries
not really user changable, you risk killing it. or you spend a lot of money to get someone to have it fixed (so far i only know ipod has "changable batteries" and none for Dell DJ or Rio Karma) *i dont want to buy something knowing that it will be a paper weight once the battery dies
-harddisk
mechanical drive, tendency to fail, can't really take drops and too much bumps. if it dies you losing gigs of music
-size
still larger/heavier than MD players
MD Players ->
+uses MD
music is safe, durable
+battery
depending on model, some runs on gumstick and some on AA, some runs from both, easily sourced & easily changed, most have 10hrs of playback, new Hi-MD models have 16hrs of playback *NH1 model and combination can go up to 30hrs+
+in theory at least, these things will last longer than HD players..since HDs do wear out.. i know my discman by sony is still going strong after so many years..but i know i have had a lot of HDs that died on me
== small, but you will have to carry extra MDs for more music
-MD size, 1GB
as of now the Hi-MD are only 1GB, but i do believe that it will increase further in time.
-new Hi-MD players also act as a portable disk drive, but it has a limited bandwidth of 9.8mbps - roughly 1MB/s which is slow (USB 1.1)
-ATRAC only. unable to play other formats. mp3, wave, flac etc will all be converted to ATRAC. i believe when converting an MP3 -> ATRAC will be worst than converting from CD -> ATRAC as mp3 is a lossy encoding and so is ATRAC.
-copy protection can be a PITA
-have to carry more MDs if you want more music
now if only the Hi-MD players had USB 2.0 and a higher transfer rate, plays MP3 (since you can use it as a portable disk drive, you can store mp3 on it, only if you can play it. - then again ATRAC is sony's proprietary codec and they get paid for it so they would want to force people to use it) and has 4GB disks or larger..then its a winner in my book.... USB 2.0 will come, and i believe 4GB disks as well..but playing mp3 is something that will never happen...then again 2 out of 3 is good enough for me IMO
i'm still leaning towards the MD as i know it won't turn into a paper weight if i accidently dropped it, or use it for a few years. my friend has a MD player, 94' which is still going strong today..
the only problem is i can't wait till they come up with USB 2.0 and 4GB disks on the MDs....
decisions..decisions..