OK, You've basically got 3 sorts of players, all revolving around the storage method.
1 Hard drive based players hold the most tracks, but chew up battery power fastest and are the most fragile. These things can hold up to 20GB of files.
2 Next up is disk based ones (CD or MiniDisk). One CD will hold a fair bit of music, but again they don't take well to knocks. Each CD can only take 700MB or so of files, but it's fairly easy to carry a few disks with you and change them on gas stops.
3 Finally there's memory based ones. These just use memory chips to store the files so they're very robust and good on batterys, but limited in space. Some of these players seperate memory cartridges (like the ones you see in digital cameras) so you can carry more music and change it round as you go, but memory's expensive.
The storage space is not the only thing that affects the play-time - the quality the MP3's are encoded at does too. As has already been mentioned, the better the quality, the bigger the file. MP3 quality is measured in kbps (KiloBits per Second). 196kbps is generally considered to be CD quality, 128kbps is roughly radio quality, and 96kbps is pretty poor indeed - however at 55mph you're probably not going to notice if the high notes are a little scratchy. Either way it pays to find out what sort of bitrate the player supports. Some also support VBR (Variable BitRate) which increases the bitrate in complex parts of the music, and decrease it in simpler parts... best of both worlds.
There's lots of other features/gimmicks that they throw in, like external speakers, FM radios & voice recorders, rechargable batterys etc - it's up to you to decide which of them you want.
My personal choice to was buy a cellphone that has a replaceable memory chips and can play MP3's and FM radio through the same headset as the phone itself uses. When the fone rings the music mutes and the fone auto-answers. When the caller hangs up the music starts again. Having said that I've found it's usually more bother than it's worth so it's rare that I actually use it anymore.
Hope that lot helps...
Matt
