ZBrush is interesting- I wish it was more like clay. You can trim(carve) but it is more like pushing and pulling a mesh shell around. It's new ZModeller is great, really easy to use and good to rough hard surface or machined type shapes. ZSpheres are great for a base organic mesh.
3DCoat is all voxel based and is more in tune with additive/subjective techniques. All just tools to achieve the result.
Sculptris is a good intro and much less tech centric...http://pixologic.com/sculptris/
The one problem with a lot of these packages is measurements- it is possible to work to scales but very unlike CAD style software.