One alternative not mentioned is Truespace (now free). It has most of features included in XSI, C4D etc. including bones, animation, more complete modeling tools than Silo, plus a not half bad render engine. If you are interested take a look at their organic modeling videos. http://www.caligari.com/products/trueSpace/ts5/Courses/WorkspaceOrganicModeling1.asp?Cate=Training&Subcate=Fundamental
The interface is not inspiring but not nearly as daunting as Blender.