I had the 5.3 in a 2001 Silverado Z-71 and it was wonderful. I haven't any experience with the 4.8 except for test-driving one when I was shopping, and with one in a truck at work. I preferred the "feel" of the 5.3 - hard to describe, but it seemed better. When I test drove them, I said that I could live with the 4.8 but love the 5.3. Mine had the 3.73 rear. Check the rear - that's more important for towing. I never had any trouble staying in OD on the highway. Only trouble I had was the throttle body got sticky once. My mileage was 19 on long highway trips, 17 commuting to work (have a 108 mile commute), 15 around town, and 14 or so towing my 3000 pound boat (some extra idling at the boat ramp). I always used Mobil 1 synthetic, so maybe that helped the mpg. It towed better than the 5.4 in the Ford I now have.
On a used truck, I could live with the 4.8, but buying new, the 5.3 seemed worth the money. I believe the 4.8 is evolved from the old 283 which was a fine, reliable engine. If the 4.8 is paired to a 3.42 rear, you should get slightly better fuel economy - maybe break 20 on the highway.