If you delete the AFM or whatever the cylinder deactivation is called the 5.3L is very reliable usually. If you run it as-is with the 4-8 switch you'll get slightly better mileage but may need an engine down the road. 

I have a 2011 GMC w/ the 5.3L and disabled the AFM at about 80k. Love the truck, rust is just starting to show now 14 years later in the rear fenders where they all do. I realized too late you take the tail lights out to really clean that rear fender/bed area well. 

6.2L has more power of course and somehow gets almost as good of mileage as the 5.3L. 

If I could do it over again, I'd get a 6.2L without cyl deactivation