I would go with the 1500 as it will do everything you need and will probably go to 200k with some TLC. I had an 07 2500 with the same drivetrain and never got more than 14 MPG hy empty and 8-10 city. I have no experience with the 5.3L 8 speed, my last 1/2T was a Tahoe with a gen 1 5.7L and 3.73 gears it got around 19 HY and 15 city - just don't expect it to tow 6,000 lbs.