neither
ihp-100/ihp-120
better sound quality
lcd remote
fm tuner
ogg vorbis support
battery life is *twice* as long
here's a cnet review of it:
http://reviews.cnet.com/4505-6490_7-30571493.html?tag=prmo2
you'll notice it has the same rating as the 20 gig ipod, about same price.
and as for the zen... i dunno, maybe it's just me, but i don't like to buy stuff from a company that makes you wait in line just to download sound card drivers.
basically the only reasons that you would get an ipod over this, that i can think of is...
1. ignorance
2. ipod is a hair (really a hair) thinner
3. you have an aesthetic preference for the ipod (which is fine, it's a personal thing)
4. you value little things like the calendar on the ipod
5. you value a relatively large third-party accessory market