mudbox is great if you use other autodesk programs like max or maya, as it opens directly within them and cuts back on your workflow. Zbrush is standalone but has more tools and features but is much harder.
There was a time when they were both very equal in capabilities but zbrush has grown a lot. Unfortunately, it has also grown in complexity, going from "strange and confusing" to "wtfhowdoyoueven" levels.
Hahah. I don't use Mudbox for sculpting. I never figured that out. Right now I'm all about it for texturing and yeah, like Max says, it helps so much because I primarily use Maya
i got through about the first 30 vids before I had to stop paying for more time. What I did learn was extremely useful/well done, however. They have SO many tutorials. Next best thing to taking an actual class.