Kinda depends on whether you like Illuminations or Wishes better. This past NYE, MK showed Wishes at 9:00 p.m. and then their old fireworks show (can't remember the name already) at midnight). Both were spectacular. We loved them. (Watched Wishes from the deck at the Contemporary after dinner at Chef Mickey's and the midnight show from our balcony at the Polynesian.) The NYE Illuminations is also spectacular (better even than the normal version). But similar.
One thing to take into consideration, though, is the crowds. If you're going into MK, plan on going at park opening on NYE and staying ALL DAY. The park will close. (This past year, I think it was closed by 10:00 a.m.) Epcot is more flexible usually, though it has been known to close, just not as frequently or as early.
We don't usually do a park on NYE when we go. We just do other things all day. (Went miniature golfing this past year. We wished we had planned a day in the water parks. It was HOT!) Then we book PS's someplace fun and enjoy a late dinner, go back to our resort and watch the fireworks from there. (We try to stay at a resort with fireworks views for NYE.) But we have a 9 year old, and staying in any park for more than 12 hours would not be a happy way to start any new year. It might be fun under different circumstances.
Julie