What is the theoretical difference in asteroids that explode before impact and the ones that impact the Earth?

Air resistance (actually, any resistance to an object's movement through a fluid/gas) depends largely on surface area. If the asteroid separates into smaller pieces before impact it will increase the air resistance, which will A) cause the pieces to burn up faster and B) slow their descent. An asteroid that explodes beffore hitting the Earth would do less damage to the Earth itself. Of course, if there's asteroid shrapnel spraying everywhere it can't be too good for the people either.