Do DCA on NY Eve. It's larger so it can hold more people without feeling claustrophobic. Be aware that on NYE, if you leave the park, for any reason, you may NOT be guaranteed re entry, even with a hand stamp. The parks close to capacity every NYE and if that happens and you are outside the gates, you are not getting back in. There will be signs up at the exit turnstiles warning you of this. So choose a park, get there early, and commit!

New Years day is a surprisingly slow day. Go to Disneyland.

FTR, it is Disneyland, not Magic Kingdom.