Zbrush > Mudbox
Sorry, I had to say it. I've been using Zbrush lately, and it's one of the most amazing programs I've ever seen. I haven't used Mudbox, but from the pics/videos I've seen of it, Zbrush is clearly the winner. Of course, this could just be another pointless maya > max debate. (coughmaxftwcough)[/pointless off topic rant]

