Mudbox's biggest strength over Zb is its 32bit ability on textures (in and out) plus its vector displacement tool...insane.

Zbrush is better for sculpting overall though............

I wouldnt say they are trying to emulate each other though, its just the usual software battle like it used to be (MAx vs Maya etc) both do the same things but in a different way.

But...... Imagine if Autodesk did an interface like Zbs...you'd all shit yourselves!! LOL

cheers
Jay