Differences can be subtle, but each dap has their strong points. Ipod is a good affordable choice. Esp for the battery life and Ui. I remember being really used to the sound of my iPod and having trouble accepting a different sound, let alone better. A song is a song is a song. A really well recorded song will will sound good on a lot of stuff. Variables include file types etc. I think there are too many variables to single out DAPs themselves without considering the synergy of all them.
I think one of the purposes of having a hobby like this is to analyze and obsess over subtle differences and improvements. Sometimes they add up to something special for the individual.
Some will always say they can't hear the difference, others will definitely hear it and enjoy it.
In other news, any recommendations for a replacement headphone jack for an iPod video 5th gen 30gb? Will any do?