This is getting a bit more subjective, but I much prefer the Zune Marketplace. The interface is colorful, has more flair, and some cool features like 'Mixview' that let you quickly see related albums, songs, or other users related to what you're listening to. Clicking on one of those will center on that item, and another set of "neighbors" will come into view, allowing you to navigate around exploring by similar artists, songs, or users. Speaking of users, the Zune "Social" is also great fun, letting you find others with shared tastes and becoming friends with them. You then can listen to a playlist created based on an amalgamation of what all your friends are listening to, which is also enjoyable. Those concerned with privacy will be relieved to know you can prevent the public from seeing your personal listening habits if you so choose.
If you're still on the fence: grab your favorite earphones, head down to a Best Buy and ask to plug them into a Zune then an iPod and see which one sounds better to you, and which interface makes you smile more. Then you'll know which is right for you.