Bear in mind the following is all coming from a VERY novice 3d user :D;
Sculptris is a great tool initially, but it crashes way too often for long term use! This seems to vary from computer to computer, though, and seems to be independent of system performance. I can’t seem to get above 500K polys before it refuses to do anything, but I’ve seen computers lower spec’d handle much higher poly counts no problem.
Autodesk’s Mudbox seems to be more stable, and is free with a student license. Sculptris was bought up by the Zbrush guys, so the next iteration of zbrush should have the same ease of use implemented in it. It’s such a great way of sculpting.