Zbrush is primarily used to add super-details to objects, although it does have the functionality to create objects from scratch. It just isn't necessarily the best at doing so and uses it's own method "zspheres" that isn't done in any other app.

So, the real question is "which is better, Maya, Max, or Lightwave?" but then that will start a 80 page discussion that will all culminate into the answer being "depends".

But between zbrush and maya, Maya can be summed up by saying you can make anything you want. Zbrush can be summed up by saying you can detail anything made in Maya out the wazoo. Zbrush is a companion app as opposed to an app to buy "instead."

hope that helps a little.