I have done MGM and MK, and I have to say MK is the best. There is way more to do there and their fireworks are a million times better. Get there at opening, get lots of fastpasses, make lunch and dinner ps, and be prepared to spend several hours online or just hanging out. By about 9/10 pm we were pretty much done with all the rides we wanted to go on and ended up playing cards in cosmic rays for an hour or so before staking out our spot for the fireworks. by far the best new years ever.