Monkey Grip enables you to wield a larger weapon at the cost of -2 to attack.
In the best case, a larger weapon gives you 3.5 extra damage: Greatsword 2d6 -> large Greatsword 3d6. The increase is even lower for smaller weapons.
Power Attack gives you more damage increase for -2 to attack, and it is more flexible, as you can change your attack penalty and damage benefit from attack to attack.
Why would I ever pick Monkey Grip?