How big of a truck for safe and comfortable towing?

My research on 2015 Chevy Duramax 3500 4x4 weight ratings for a possible future purchase is this -

* Crew Cab Long Box DRW
Max payload 5,010
GVWR 13,025
RGAWR 9,375
5thwheel rating 22,800
GCWR 30,500

* Crew Cab Std Box SRW
Max Payload 4,020
GVWR 11,500
RGAWR 7,050
5thwheel rating 17,100
GCWR 24,500

Of course it will vary based on other body styles.