You're planning to tweak the model at +1,000,000 polys. Good luck with that! Give us a shout next year when you finish it!
Both Modo and ZBrush would be great for quickly sculpting something like those images. But honestly, what you get from this technique can produce an unnecessarily heavy mesh. You will have unnecessary geometry where you don't need it. And you will have not enough where you really need do it. So depending on what your design is, you might get better results by modeling it. It will just take longer to do it. And then of course you have the choice of Nurbs versus polys. I would probably use Nurbs (in Rhino). At least this way the final design can be optimally meshed before generating the toolpaths. If you create this as a sculpted mesh or subd, then you are stuck with a set # of polys. With Nurbs you would be free to remesh the object at any time, for best results on different size work... Also if you have Rhino 4 (do you?) then you could sculpt a grid in Modo or ZB, export that displacement texture and apply to a NURBS surface and extract a mesh at any density you need for CNC...
shorty
PS - real men sculpt stone! Isn't that right, Couerl?