The Zune concentrates on being a Portable Media Player. Not a web browser. Not a game machine. Maybe in the future it'll do even better in those areas, but for now it's a fantastic way to organize and listen to your music and videos, and is without peer in that regard. The iPod's strengths are its web browsing and apps. If those sound more compelling, perhaps it is your best choice.
I'll gear this review to 2 types of people: current Zune owners who are considering an upgrade, and people trying to decide between a Zune and an iPod. (There are other players worth considering out there, like the Sony Walkman X, but I hope this gives you enough info to make an informed decision of the Zune vs players other than the iPod line as well.)