Having used both Mudbox and now starting out with zBrush, I have to say zbrush is far superior in wot it does and wot it provides. I could go on all day about how wonderful and scary it is, but I may fall asleep.

I had the attitude of being scared of the UI in Z (dumb i know), but its pretty intuative and easy to pick up once you have gone over it. Mudbox is good dont get me wrong, but Zb wins out for me. A superb app with great control, Il be using in more and more when I get to grips with organic modelling.