Welcome to the forum:
As a general rule, the smaller the liters, the better the mileage. However, this is hard to answer if we are not comparing apples to apples. I mean take a 6.0 that is brand new and offered with the AFM system, VS a 5.3 that is four years old and not equipped with one. The 6.0 will offer more power and better gas mileage.... so hard to say. What are you looking at? A new truck? Brand new or pre-owned? Perhaps a bit more info may help to narrow a better answer for ya.