DL has the fireworks at midnight, and in front of the castle is a very popular spot to be (like you have to stand in standing room only space for 2 hours before midnight). It get's packed, but it's nice to see. DCA doesn't currently have fireworks (although I've heard they will have a new show that does have fireworks). DL will probably be more crowded. Even if DCA is filled to capacity it's still a lot less that DL filled to capactiy. I suggest making sure you enter the park before early afternoon, they've been know to close the part to new guests the closer to midnight it gets. Traditionally DCA opens later than DL (usally an hour), and closes earlier than DL (at least an hour, sometimes more). DL is usually open until 2 or so on new years. Since it's the first year, no idea what they will do. On Fourth of July they had electric parade at DCA and when it was over there wasn't enough time to get into DL for fireworks, but most people sat in the big circle area between the two parks and watched the fireworks there.