my brother has a sony MDH5 (or something like that) and it has lasted him very well - the best thing about it is that it has a removable battery. My new 1GB Ipod Shuffle has to be replaced for new when the battery fuks up. On the other hand his mp3 player is 40GB, but i go running with mine so it had to be extremly light and be able to clip onto my bag strap and not bounce while running.
Benifits of MP3 - usually cheaper, replacable battery
Negative - possibly doesnt look as nice, not as many accessories
Benifits of IPOD - looks more aesthetically pleasing, and is the "fashion" item at the moment
Negative - Battery cannot be replaced, more expensive that other mp3 players
work out what you need it for, and how much music you want it to hold - then base your decisions on that