all 3 players are quite mature models, so i suspect that for some reason you have no or little access to more modern players, so i wont start with "you should buy this or that", im just gonna give you the low down on all these players,
i had a NW-HD5. theres a program called MP3filemanager that installs directly on the player, it makes the player +/- drag and drop under windows, so you can avoid sonicstage,
the gigabit S60 is the worst of the bunch, worst sound qaulity, lowest battery life, and worst managment software,
the cowon is the best in sound quality, file support, but its library managment is peculiar, im not sure if they fixed the mp3 tag issue, theres also the little detail that you have to have that minidock to connect it to the computer, charge ir and use line out, which is a bit annoying,
the NW-HD5 offers great sound quality, great battery life, has line out by software (turns on and off the amp), but has poor file support and no lossless support, using MP3Filemanager you can decrese the damage done by atrac and sonicstage,, its also the most compact of the 3 and has a user replaceable battery (without the use of a soldering iron and a set of screw drivers)
now, becuase i owned the HD5 i can tell you a couple of secrets,
the HD5 hard drive can be upgraded to 40gb if you have the right tools and stedy hands, and the battery can still be obtained new from several online stores, so if you´re buying a 2nd hand one, its easy to refirbish, atraclife.com has the full service manuals for them, i have done the research towards this and have posted my findings at atrac life, but unfortunatly, a friend of mine needed a mp3 player before i could start working on it, and my friends are more important then my ego, the control remote can still be found cheaply on ebay, just be carefull as older remotes tend to give trouble with the menu system, the player is very sensative to cold, i had the hard drive stop on me when outside at temperatures under 5 degrees C, and ALWAYS and i do mean, ALWAYS, have a paperclip taped to the back of the player, just in case you have to remove the battery,
