If you really want one of the two, you should probably think about the stores associated with them.  The Zune store is an all you can eat buffet of songs and the iTunes store is a la carte.

 On the computer side of things I would imagine you're using Windows, as the Zune doesn't work on OSX as we type.

 If neither of those are a factor, I'd suggest heading out to a store and give both a  try in terms of how they feel in your hand.  They're controlled differently and each has their own fans.