I have to say that US is waaaaay more user friendly than MGM. MGM makes it almost impossible to see multiple shows and still enjoy the rides. We did a lot of cutting back and forth across the park and still failed to see all we wanted to see (and we spent 2 half days there).
IOA and MK are apples and oranges. MK is my favorite park, ever. I liked IOA but it was different. Good, but different. Not as much for very young children.
But I also want to add that in comparing Disney and US/IOA, my children were able to more easily access characters in the parks at US/IOA. Just one of several positive differences.