This is a great question and Kenny prefers both. In this tech video, he explains why he uses one setup versus another. On high-powered or supercharged Mustangs you want more tire at the rear, but on a well-balanced car with moderate power the square setup makes sense.
The cost difference is always a concern with staggered wheel and tires, or you can use the same wheel and tire size on all four corners and save some time and cost. In many cases, you'll need more rear tire for traction so always consider that when building your car.