My personal opinion:
Stay far, far, far, far, far, far, far, far away from MK. It's insanely crazy. However, if you do decide to go, getting there early should get you in with no problem. MK shouldn't hit phase closings until 10:00 am or so. And then, if you're a resort guest, you won't have a problem getting in up until phase 4. But, it'll still be crazy.
Epcot is just as crowded (actually moreso), but its huge size doesn't make it quite as bad. It's still the most crowded day of the year.
There are some pretty spectacular fireworks displays during NYE, especially at Epcot, where each pavilion has its own mini fireworks display, plus the mega-huge grand finale tag.