I think it boils down to some various choices. Ipod, for sale from both HP and Apple, uses the Apple music store for online music downloads and supposedly makes it difficult to pirate music (read that steal). You can also upload your own CDs into your playlist and transfer them into your Ipod. From what I can see, all the other various units are locked out of the Apple music store, which is supposed to have the best selection. The others also offer capabilities to use the Roxio on line music site (formerly known for prirate downloads but now legitimized after being purchased by Roxio) as well as various ways to upload CDs into your units.
Ipod is generally considered the best, but I suspect all of them work! Sort of like tractors. From the standpoint of longevity, expandability, and customization, nobody offers anything close to Ipod. (Bose even offers an add on speaker dock) I have a couple friends who jog with their Ipods and they rave about them.