One thing to remember, if you do plan on going to a park on NYE you have to go first thing in the morning and get your hand stamped so you can come back later.
We were at WDW this past NYE. We were told that a hand stamp will not get you back into a park if it's at capacity. Actually, nothing will get you back in...hand stamp, priority seating, paid event..nothing. As I recall, closings go in this order...parking lots first, then offsite guests, then onsite guests.
We went to MGM studios, but didn't get there till around 11 am. By that time our driver said that MK was closed to offsite guests, Epcot was nearing capacity, but that MGM was fine. Whether Epcot ever hit capacity or not, I don't know. MGM was extremely crowed in the late afternoon and early evening, but I don't believe it ever hit capacity.
The downpour hit around 8 or 9 that night and MGM was a virtual ghost town by the time midnight rolled around. It was wet, but it sure was fun watching the fireworks and walking onto the rides with no wait all evening!
I'd suggest whatever you decide to do, if you're going to MK be there when it opens. If it's Epcot, I'd get there fairly early. If you want to be in a park at midnight, stay once you get there. Unless of course you get hit with the rainstorms we had....then you can go anywhere you want!
