Usually Magic Kingdom is our park for the last day. If, for what ever reason, things don't work out for us to spend our last day in Magic Kingdom, then Epcot will be our last day.
I think I enjoy both parks for the last day for the same reason. I enjoy the fireworks being the end of the day, and the way the park plays different music after the show, as a sort of goodbye. There is just a feeling like yup, it was a great day, now it is over, good bye old friend that I get from the closing of both of those parks, and don't feel the same way at the closing of AK, or DHS.