hmmm...
they are actually to close to call for me. I have ridden in both, but have only driven the MDX.
I personally would go with the MDX because it is an Acura, but that isn't really based on anything.
They are both good suv's and I would take either of them over the BMW X5.
here is a pretty good side-by-side comparision of the two you are looking at. They are really too similar...
click here