I would definitely recommend December 30th fireworks. It's the same show and much, much less crowded. That's not to say that it won't be crowded - it will be very crowded, but not nearly as bad as NYE.
I wouldn't bother with MK on NYE - personal opinion. It's a small park and the most popular park. We've done DHS in the past and loved it. It's gotten a little old, so we did EPCOT for a little while last year and will spend the night at EPCOT this year.
EPCOT is so spread out. It's definitely PACKED - but it's easier to navigate the World Showcase than it is Main Street. Remember - you could stand almost anywhere in EPCOT and see the show. At MK, if you're not on MK, you miss a lot of the show/don't have the best angle. For this reason, I'd recommend EPCOT or DHS.
For more tips and info, check out my free guide at survivedisxmas.com!