I have a Cintiq 21UX, I also have an intuos 3 (9x12) and use it with my other monitors. I prefer the Cintiq for ZBrush, Photoshop, Illustrator. The Intuos I use for After Effects, Premiere Pro, XSI, and my other software. I vary rarely use a mouse.
If you want portability the Wacom Companion looks good and some people report good results with ZBrush. Although I have not tried it, I suspect it might have some limitations given only 8 GB of RAM and not upgradable. That has not stopped me from contemplating getting one since you can take it anywhere including client meeting etc. Currently they are offering $395 off the regular price (not a bad deal) but it makes me wonder if they are getting ready to release a new version.
All that said if you are not doing this work professionally (getting paid) you may want to stick with an Intuos initially and go for a Cintiq when your resources or paying gigs justify the expense. You might be able to find a used one at a reasonable price too.