I also did EPCOT last year for NYE. There was plenty of entertainment and 2 Illuminations that night.
I have read the the fireworks at MGM this year were very good. Illuminations was good, but not as good as I was hoping for (it was just the holiday version, nothing else)
My thoughts are that EPCOT is far better at handling crowds because of the extra space. Most of the time, I did not feel overwhelmed by the crowds and used the time to explore the little known features and shows.
I can see MGM being a difficult place to navigate if it is at capacity.