I’d say Mudbox and ZBrush are basically redundant for the most part. ZBrush has more tools, and is much more convoluted to work with than Mudbox. Mudbox is simpler, but has it where it counts. ZBrush has a lot of stuff you simply do not need if you use a base 3D modeling application. I prefer ZBrush’s canvas navigation to that of Mudbox, because it it more friendly for working with a pressure sensitive Wacom Tablet, (which is essential btw). You only need to use the pen itself and any other clicks are done on the keyboard to change between pan, rotate zoom. Where as in Mudbox, Maya’s 3 button navigation which it clones is a pain at first using a stylus, but I have gotten used to it and you can remap the keys if you like. The camera itself however, is much nicer in mudbox, and functions more like a camera in a base 3D modeling application.
Mudbox has better previewing options, especially if our outputting for games, to get a feel what the model will look like in engine prior to output. ZBrush has a lot more brushes by default, most of them however can be re-created in Mudbox if you know how to work with your brush settings.
Mudbox is more reliant on your video card, ZBrush is more reliant on your RAM. Thusfar, every new version of ZBrush has been a free upgrade to previous owners all the way back to v1.0.
There is likely nothing, if you have a base 3D app in the equation here, that you can do in one but not the other. So it will come down to preference. And as someone mentioned earlier. Mudbox is 1000x easier to learn, and it’s new features tend to be easier to absorb. Mudbox is also friendlier with dual monitor setups as it allows for floating windows and pallets that you can put on the second screen.
My vote hands down goes to Mudbox but you honestly couldn’t go too wrong by getting ZBrush instead, and Pixologic is a very nice and hard working company, as well as brilliantly creative and always listening to it’s users needs and wishes.