Think about it in terms of stroke length. A 5.7 has a stroke of 3.48". A 5.3 is 3.622". So the 5.3 has the advantage right out of the box. All things being equal, the 5.3 will have more torque. But things arent equal. The 5.3 has better heads and a better fuel injection setup so its better still. And if you are going to go through the trouble of swapping engines, and arent willing to put a cam into it that addresses your specific needs, then why bother.