Your comparison was to be between either Epcot or MGM, right?
First and foremost, the fireworks at Epcot are unbelievably great. And, since Epcot can handle a larger crowd, and it will be crowded everywhere, I would go to Epcot. One year transfers between parks were restricted after a certain hour, I believe 6pm if memory serves. I recommend around 11 pm going to the American pavilion at Epcot. It will also be crowded, but you can get a seat, rest awhile and then watch the fireworks.