I've only used mudbox and think that the user interface is simple and very user friendly. You can be up and modelling some pretty cool things in no time at all.
The latest ZBrush 3 has lots more options and has realtime painting on the geometry which will be a big plus for a lot of people. Mudbox 2 will be out this year sometime and it sounds like it'll have a similar 3D painting option with lots of new tools too.
I'd say run the demos and see if they float your boat or not

Si