Zbrush, if you want alot more detail. It can handle a much higher poly-count. But the learning curve is kind of steep.

With mudbox, its more of a traditional layout, with almost no learning curve, but it cant handle NEARLY as many polygons. I personally would say to go with Zbrush, because its brushes are much more customizable, and you can texture and make normal maps right in the program. Mudbox doesn't do that.