Epcot NYE is a blast! Best party in central Florida by leaps and bounds. Flame throwning dance floor in China, EDC in Italy, family dance DJ at fountain hub, some years they have had a silent disco near Japan. Its all what you make of it. Both Magic Kingdom and Epcot will set up many temporary dance floors. Due to its overall larger size Epcot will be much better to able to handle the crowds than Magic Kingdom will be. Magic Kingdom will literally be wall-to-wall people. Epcot will have parts that get bottlenecked with wall-to-wall people but for the most part you will still be able to walk around. Also the midnight fireworks at Epcot are Bar None the best display Walt Disney World does. Way better than Magic Kingdom IMO it's not even close. The perimeter is nice at the Magic Kingdom but it does not match the Firepower that is used at Epcot on NYE. It starts with Illuminations and the after Illumination the individual countrys throw a display of fireworks in the order in which they ring in the New Year. The last three, Mexico, America and Canada combined for a countdown finale that will likely be the single largest display of Firepower you will ever stand this close to.
There's also something very special about partying around the world. That's said, I will warn you that around 7 p.m. Epcot will stop being Epcot for the remainder of the evening and will start just being a party. They kind of let the countries relax their themes..... There is no reason to have an Electric Daisy Carnival party in Italy. None whatsoever. It's just there because it's New Year's Eve and it's just there because it's fun. It's the one night of the year that Epcot does not take itself so seriously and it allows itself to have a little fun.
I've never waited two hours I get out of the Epcot parking lot afterwards. But then again I do wait until the park actually closes at 1 am before I even start heading out of the world showcase. So I'm not even getting to the parking lot until 1:30 or 1:45. By that time it's pretty easy sailing out of the parking lot. I 4 on the other hand might be a different story.
So if you're willing to deal with crowds, and it will be crowded, and you can have a route home that does not involve Interstate 4, I say both Epcot and Magic Kingdom are the way to go. But I strongly lean towards Epcot.