I was looking into the Apple iPod and Nomad Zen, and chose the Zen because it was about half the price of the 20GB iPod. Brief overview of what I found and what concerned me:
iPod - noticeably smaller, lighter, and sleeker. Hands down style winner, and the touch-sensitive navigational wheel works like magic. Good OS too.
Zen - Half the price of the iPod, and very good looking, too. Better headphones, if you don't have your own - I use them when I jog or rollerblade cause they don't come off and cause I can hear cars coming and people yelling at me, unlike my Etys. Works with MP3s and WMAs (these really do sound better at lower bit-rates - solid bass and crisp highs at 128bps - can't tell the difference into my Etymotic ER4Ss between this and 192bps MP3s). Compatible with Windows Media Player 9 series - which is a good thing, because Creative's software won't work once you install this (although Media Player transfers noticeably slower than Creative software, and you cannot pull stuff back off of the Zen with it, it's really convenient otherwise as a player and ripper [great CDDB support]). Freezes ocasionally, most notably when plugging and unplugging the USB chord, but easy to reset and never a real bother unless you can't find a paper clip or a staple lying around.
well I spent more time on the Zen, cause that's what I own.
Hope this helps