Final Render versioner?

Currently the major difference is which platforms you can get it on; Stage-1 is only on MAX whereas Stage-2 is currently only available for C4D (with a Maya version on its way).

Stage-2 is stand-alone - ie it does not sit within your modelling package like a plugin, rather you export to fR in order to render. The GI engine in S2 is completely different as well and can handle animations and network rendering much better. There is also a node based shader editor.

For MAX there is a Stage-1 R2.0 planned which will be the Stage-2 render core but still embedded into MAX - as has been much discussed this is seen to be the way most MAX users will want the functionality. By having it standalone you lose the option to use all non-geometry plugins (such as hair/fur, etc).