Hello,
I would go with the Zune. I played my 30GB for over 40 hours straight when i went out of town and it ran the whole time from a full charge. I was able to make it to my destination, travel around and make it back home with battery power to spare. Great battery life on the Zune.
Not to mention you can play video's, Podcasts and everything else on it.
Cheers,
Mak