Consider the options:
How many songs
Battery life
Durability
Software
Accessories
MP3 players come in 2 formats really, solid state whereby the music is stoed in a chip, so no moving parts, advantages of this are they are possibly more robust and the battery life will be longer, also the physical size should be smaller (chewing gum packet size), the downside is that there is a limited capacity and not many functions on the player, no playlists etc. Hard disk based players can store a hell of a lot more music and some have extensive features, the downside of these are moving parts, shorter battery life and slightly bigger in size.
Apples Ipod I guess is the daddy as far as design and features, but that does come at a price. I've got a mini mainly for snowboarding and they're now available in 6Gb size so probably upto around 2000 songs but iTunes (the software that sits on the PC) is great and you can create various types of playlists (Rock, Pop, new songs, new songs which haven't been played more than 5 times and have a rating of 5 etc etc)
It's small, light and the battery life on the latest models is quoted at 18 hours, and the range of accessories is huge in comparison to all of the others, car kits, wireless transmitters, hi-fi integration etc etc
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