There are several "biggie" differences with Infinite:  Painting ecosystems and python scripting.  They are only available in Infinite and open up a whole new world of things to do. 

There are many other increased improvements in the function generator, light controls, etc that are subtle, but critical when you need them.  I'll second the recomendation to try the PLE.  Once you paint your first ecosystem, you'll upgrade!

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