I can't tell you how the Dell works exactly, but I'd imagine it's not the same as Apples UI. As you may already know, iPods work with iTunes to download music. Dell players use software of some kind, and it will be included in the packaging. Considering that WMP10 works with most non-iPods, you can likely use WMP10 to manage your music on the player.
Hard drive storage is fine if you don't do many high-impact activities while listening. Walking is fine, but running, jogging etc will likely damage the HD over the long run. That's where flash players come in handy, as they have no moving parts to damage.
Can't help you on what to get, as it's really just a personal choice. Both players should do the job. iPods have a "wow" factor that most other players don't, but if you don't like iTunes, then you may not want an iPod.
My daughter really likes her Dell DJ. Dell recommended at the time of purchase (about 1.5 yrs ago) that you use MusicMatch Jukebox to organize your music, but she uses WMP and it works fine. She had a little trouble figuring out how to establish play lists through WMP, but finally got it.
I also know plenty of people that swear by the Ipod though, so I would recommend trying to find somebody that has one and checking it out (the Dell also if you can). The price/capacity is a good bit lower for the Dell DJ, but you might find that you just don't like it after using one.
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