Hmm though one, I would think MK is going to be exceptionally packed. I don't know if Epcot would be any better. However, with the whole colonial feel that America in world show case, I think that would be cool. That being said I think for me anyway, it would depend on who would be going with me, kids etc....Whichever you chose, I would plan on sticking to that one park all day and getting there pretty much when the park opens to ensure (or at least as best you can) that you will be able to get in, before the park fills to capacity.
But either way, enjoy your 4th in the world!