That 5.3 Silverado is a very solid all around truck, but if you don't mind losing the space the smaller trucks will definitely net you better mileage. I wouldn't be surprised if those 2 models are getting 18-20 if for no other reason than they're 1000lbs lighter than their full sized brothers.
People may tend to steer you toward one brand or another, but honestly all the trucks out there right now are pretty good. Good luck in your purchase
