McDonalds tends to not leave me full so much, but I think generally speaking, it's nicer.
Or, at the very least, I prefer McDonald's sides, particularly the fries. I'm also partial to McFlurry and Chicken Nuggets, the latter of which are definitely better than Burger King's version.
Burger King tends to be richer, and a bit juicier though, I think. But I'm kind of a bit uneasy on the idea of eating it now, since I quite recently severely overate Burger King, when my brother and I had it delivered. It was a big joke, but it left us feeling sick.
Must admit though, I do have a craving for their Jalapeno pepper cheese balls, even though they have the consistency of a popped zit.