I voted for the Zen Micro because although I love my Zen Xtra, I do wish it was a little bit smaller.
I bought my Xtra about a year and a half ago because it was $100 cheaper and had more space than an iPod. Since then, I have never dropped it and I've only had to reset it once. I was afraid of jarring it too much when I run on the treadmill so I used to sit it on a table beside me, but now I just put it in one of the holders and jog away. It's still a pretty reliable device that sounds great.