XSI Foundation is pretty awesome for the price as it has all the same modelling and animating tools that it's advanced version does. Even has cloth and particles and you can buy a hair plugin for another $100. It can do mixed tri/quad subds so that tris get divided into tri and quads to quads all on the same mesh. And of course you get a licence of mentalray which is a pretty nice renderer.

But if you want a pure modeler then perhaps modo will be better for you. Its animation and rendering suck in comparison but it has a couple of tools that XSI doen't like for texturing and sculpting.

Personally i use XSI + ZBrush, but that comes out more expensive then modo alone and not everyone likes ZBrush's GUI.