I did MK for my birthday - for me, it's always my favorite because it has all of the classics. Plus, I ended up getting the Birthday Fast Pass and felt that MK would be the best place to use it. (In the end, the crowds were much lower than I was expecting and we didn't really need the BFP - I should have got the gift card instead.)
Wow ~ my birthday is on December 10th too!!! I voted for the Magic Kingdom because that is where I will be spending my birthday too!!! We have tickets for Mickey's Very Merry Christmas Party that night!