My understanding is that alot of the features in pro that are not in P8 are to do with the rendering engine, not it's quality just some extra features.

Rendering in a background process
64 bit rendering

and probably a few others I can't remember :) there will be a new version of pro later in the year I think. 

There are some new features in P8 that aren't in the current version of Pro, like Indirect Lighting and the new rigging system.  And it comes with a whole host of new content, including new figures of course :)

John

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