19 has the 10 speed instead of 8, and DFM instead of AFM. Overall the 19 is much better driving truck dynamically. The powertrain and handling is perfect. Probably the biggest reason to go with the 19.
You also have the option of more goodies in the 19. HUD, 360 camera, rear camera mirror, trailer tire pressure monitoring, heated rear seats with seat-back storage, multi-pro tailgate, deeper and wider bed, etc. That said, you do lose power pedals, height adjustable seatbelts, sunglasses storage in the overhead console and some other little things they deleted for 2019.
There are some definite reasons to consider the 2018 too besides price. Being the last year of that body style, most issues were pretty well sorted. Save for the 8 speed that some people hate and maybe the chance that it’ll have the Chevy shakes going on.
The 2019 is a first year truck so there’s bugs and glitches. Fair chance the rear window will leak (or has leaked) on the 19 too, especially if it’s an early build, say pre-11/2018.