Depends on more than a few factors. The Zune is cheaper ($200 Vs. $250 for the 30 GB, which is as big as the Zune gets to my knowledge) and has a bigger screen, so if you're going to use it for movies too, it's a bit better the option. If you've got a large amounts of MP3's and you want to get 'em all on there, the iPod's really the only option. $350 for the 80 GB iPod is more memory for your money in the end (Almost triple the space for just shy of double the price), but if you don't see yourself ever filling it, it's a waste of money (I filled mine up a while ago, I have about 65-70 gigs of music and a lot of video).
The softwares are each comparable. I find iTunes significantly more user friendly and intuitive than the Zune software and the Zune has about the only music program I've ever seen that's actually slower than iTunes. Zune's software does have the advantage of letting you sync to an Xbox 360 if you have a wireless connection through your house, though, so if you have one, it's very useful.
Verdict: If you have less than 30 gigs of space needs and don't see yourself going over in the next year, go Zune. Otherwise, splurge the extra cash and go iPod.