Anything a 5.3L can do, a 6.2L can do better.
With the correct axle gear(s) & tires, so long as you're willing to pay for 91 octane, you won't lose much if any miles per gallon, and you'll gain quite a few smiles per gallon.
If the cost of gas is not an issue for you, get the 6.2. It is truly a fun motor. If you don't get enjoyment out of driving (in other words, if you just use the truck to get from point A to point B), then the 5.3 would probably be fine for you.
Honestly, if you are doing any towing/hauling, I'd get the 6.2; I actually think the fuel economy towing is better w/ the 6.2 than the 5.3 (assuming you are north of towing 3500lbs).
Its a much better driving experience w/ the 6.2. And, I even think Highway the MPG's may be equivalent or even better w/ the 6.2 than the 5.3 in similar conditions...esp on interstate at say 70MPH+.
I don't think the 5.3 at 355 HP is a bad choice...but the torque in the 6.2 and the additional HP is awesome. The down side is running premium...which is a true cost delta.
Don't sleep on the 3.0 Duramax either...that is turning out to be a fine powertrain...
Suggest driving all 3 of them...even if it is in a Pickup...then judge for yourself.
You will spend more in Fuel in the 6.2, due to the upcharge in premium fuel.
I loved my first 6.2 so much, I bought another one. This, after owning a Suburban with the 5.3. I don't mind paying the price of premium fuel, so for me it's worth it. Best towing rigs I've ever owned. The 6.2 turns the truck into a beast!
We had a 5.3 in our '11 suburban and now have one in our '21 yukon and both have been more than adequate. Don't really tow with them, although I have towed quite a few cars on a flatbed trailer and they both did fine, but mostly bought pretty much just as people haulers. The biggest difference between them I think is the 10 speed transmission on the new one makes for a much better driving experience.
I’ve had both. 6.2 is a lot more fun, especially after adding a Borla exhaust. My 2004 5.3 is rock solid engine, but poor MPG with a 4 speed transmission. I don’t drive much and don’t mind paying for 93 octane now. 6.2 maybe the last V8 we’re able to buy, go big and go loud.