My two cents...
If you want the greatest selection of accessories, the iPod cannot be beat. Everybody who makes MP3 player accessories makes them for the iPod. It is much more difficult to find Zune accessories.
If you like the concept of renting unlimited music for a monthly subscription fee, the Zune is what you should buy. But, once you do not want to pay the monthly fee, your music is gone.
I prefer to own my music. I buy CD's and rip them. If there is a better format down the road, I re-rip them. Plus, I like to own the physical product with liner notes, etc. Although, I have purchased a few tracks in the iTunes store (some with, some without DRM) and a few others on the Amazon (DRM-free) MP3 store.
In the end though, they are both very good music players so there really is no bad decision.