From: Mosquito Bite, Texas
I also own all three of the genre.
I think CogEE is far more than COG "with a couple of features" and "polishing/tweaking." It's got dozens of improvements to a pretty good game system in COG. EE is bigger (more provinces), smarter (more balanced economies, improved AI, especially in HW), more well developed (more infomration screens, more battle types, simple economy option), and with a naval system that, while not perfect, is certainly fun to explore. It's about as much as could be squeezed into an existing engine, at least in a reasonabmle amount of time.
Now, I still like COG. I played it one weekend not long ago, but the differences from COGEE are substantial.