Zbrush is more of an all rounder, you can model, sculpt, texture and generate maps, as Andy said it can handle more polys BUT, its quirky, things dont work as you would think they would, thats where mudbox comes in as its got a very "maya" control system to it and its very easy to use.
Get the demos of both and have a play around to see which you prefer
"No pressure, no diamonds" Thomas Carlyle