I also think it depends on the sexes. I have a boy and girl separated by two years. They basically have nothing in common, argue a lot. Juggling their different schedules/interests (they are in middle and elementary school now) is a challenge.
However, I also have a second son, two years younger than his brother. The boys are inseparable. They play together constantly, want to do the same sports, go to the same summer camps and want to see the same movies on a Friday night, etc.
So the older boy and girl - I'd say having two is harder. But when it comes to my two boys - having two is easier!