Go with the 6.0L Vortec gas engine (which did  NOT have cylinder deactivation and was port injection only, no GDI) mated to the 6-speed 6L90E automatic transmission (a beefier version of the 6L90).  2015 is the beginning of a new generation and you get more tech.  My preference would have been for a 2014 model year 2500HD.  But overall those are well made trucks compared to the 1500.  Not sure which options that particular one came with or what other trucks you are looking at, but I recommend you get a 2500HD that is lightly optioned (or has just the packages you need, like a work truck) - those will last longer than ones that have every bell and whistle, another reason I prefer the 2014 (that also did not have any of the driver assist technology that 2015 and newer had).  

https://m.youtube.com/watch?v=xaUuMqCcMkQ

https://m.youtube.com/watch?v=-tc7aG4WifI