Well that's a pretty difficult question, as no one has done a dyno on both on identical trucks. The Banks is a straight through muffler with a sound suppressing material like steel wool or fiberglass, similar to a glass-pack muffler.
This image shows a cutaway of a stock type muffler on top and a banks type on bottom:
And the Flowmaster uses traditional type baffles, but arranges them so the gases flow more easily. Here's a cut-out of a Flowmaster 70 series:
I prefer the sound of the Flowmaster over a straight through exhaust. Straight through glass-pack type mufflers will also collect moisture, sorta like a sponge. Many companies are now using stainless steel batting to prevent rust, and it should last longer than fiber-glass.