I just bought a 2015 Silverado with the 5.3L. It is a very nice truck and has more horsepower and torque than the 6.0L in my 2005 Sierra. The 6.0L did a great job towing our TT at 6,000 pounds. I expect the 5.3L and 6 speed trans to do a better job with it. I've only had it a few weeks and I'm averaging just shy of 20 MPG.
I did have the dealer do a locate for the NHT package. There were two in Louisiana. It wasn't worth the expense of having it delivered on top of the additional purchase price. The 6.2L is just as difficult to locate in our area.