iPod, and I don't think there's really any comparison.
Off the top of my head: (I haven't read the above posts yet, either)
1. I've had lots of experience with M$ products in the past. On every occasion, they sucked. I wouldn't take a chance on another.
2. I want to make sure I can play my tunes in my car without wasting lots of time trying to find a little-known product that works good.
3. It is important to me to have enough accessories to adapt my music play to the world around me: I have no clue what you can do with a Zune - However, I know exactly what brand to look for to turn my iPod into a digital recorder.
4. I like running. Zune + Nike? I don't think so.
5. I don't know a single person who has a Zune - therefore, no one to squirt with. (Or is it squirt on?)
6. I tried the Zune software and it crashed my computer (WinXP no less) - three times. I gave up in the end. iTunes just works
7. I've already got my music library in iTunes/iPod. I'm not wasting time changing simply to test whether the Zune really is any good (which I doubt... see No. 1, above)
8. With my iPod, I am more culturally in-tune. Small reason, but a reason nonetheless.
9. I am confident that Apple is committed to quality - if I have a problem or if the software has a problem, I know it will either be resolved in the next update or at the very least customer service will be the best available.
10. iTunes is the go-to resource for movies, TV shows, songs, etc. No other store can match it's ability all the way around.
Edit: and No. 11: I'm addicted to the little scroll wheel and CoverFlow.