Another vote for: Neither!
I currently have a Sansa Fuze. Had it for about a year and half now, still works and looks new. I love it, it's awesome and yet costs half the price (or less) of an iPod.
Samsung makes some great audio players, I've used several of those. Especially if you demand exceptional sound quality.
The iPod is over-rated. I have seen so many of these break, especially the newer iterations of the last several years.
Don't know much about the Zune, except Microsoft sells it.. and if another company's product is comparable, I'd rather buy that.