Last year I purchased a 2019 RAM 4x4 Limited Crew Cab 6.4 Gas....I sold it back to dealer months later as the AC was unable to get vent temps below 58 degrees. Some of the vents would only go to 60 degrees. RAM was unable to do anything about it and they tried a number of times. In Texas that is not acceptable. On very hot days my whole family complained about the cab temp being uncomfortable. So I purchased a 2020 GMC 2500 4 x 4 Denali 6.6 Gas. AC is awesome. Vent temps 39-40 degrees in minutes. No comparison. I know this AC problem is fairly common on the HDRAM forums so caution to make sure if you go the RAM route that it cools well on a hot day. I don't know about the 2020 RAMs. But to say all that here are some of my comparison thoughts on the two vehicles. Lets set the AC aside. GMC hands down wins that.
Interior is truly a winner on the RAM. Screen, design, (even liked the shifter knob instead of standard lever but that's preference). The Denali is nice and has 8" screen and similar tech that works well but not quite the wow factor as the RAM.
Sound system is outstanding in the RAM better than the BOSE in the GMC but that is good as well. Speakers everywhere in RAM.
Seats are a bit more comfortable in the RAM but the ride quality is a bit better in the GMC due to the independent front suspension so not as much bouncing around. Still bounce unloaded in GMC but not as much as the RAM. The RAM unloaded had significant bounce so a bit more difficult to focus on the 12 inch screen.
Headlights in RAM are excellent and move when you turn left or right. Great visibility, better than the GMC. GMC is LED as well and good but RAM is better to me.
The tech in both are similar, GPS are easy in both and work very well. Both you can monitor with an iphone app, both have all the trailer towing settings. Blind spot indicators, lane departure and sensors. The GMC has trailer tire monitoring built in and came with 4 sensors for the trailer. RAM did not have that at the time but I believe it is on the 2020 models. The GMC has a unique HD camera rear view mirror and hologram heads up display which I find very useful. I really like that feature on the GMC.
The tow mirrors are all power on the GMC and only fold in/out on the RAM I had to manually deploy to trailer towing. GMC mirrors are larger and easy to use but do look a bit like a lego but I like them in trailer mode on the GMC better than the RAM.
Engine they both power and drive about the same to me. Sound similar, can't tell much difference between power and the 8 spd and 6 spd. Similar gas mileage. The RAM had the ability to not use some of the cylinders so a green leaf light would come on when you were driving eco style. The GMC all on no variable cylinder usage. So the RAM was like a video game how long can I keep the green leaf light on for maximum gas mileage....:). So the RAM I could average maybe 1 or 2 mpg better but overall not much difference. My GMC around town I average a little over 12mgp. My RAM about 13mpg but overall they really felt and behaved the same.
Truck size is definitely different. The GMC is longer like the mega cab but the space is distributed across the entire truck so a little extra in the back seat (My son really likes the extra inches in the back seat of the GMC crew cab compared to the RAM. The bed gets 6" more is a bit deeper and to be honest the multi pro tailgate has come in handy acts like a set of stairs. I did have RAM boxes on my RAM which we really enjoyed. Great for camping, grocery store, gear...waterproof and lockable but at the expense of bed so depends on what you want to do with your truck. Payload is by far greater with the GMC and that was important. The RAM was about 2300 and my GMC is 3200. We wanted to pull a travel trailer with a hitch weight of about 1000 lbs so the GMC gives us a lot more carrying capacity in the bed when there are 4 people in the cab. We really like that. But the smaller RAM was a bit easier to drive and park around town and it has taken me a bit longer to adjust to the larger truck.
Overall both trucks feel quality and capable, look good, comfortable and can tow and haul a lot of stuff. If the AC was not an issue I would have kept and been happy with my RAM. Knowing what I know now I might have purchased the GMC over the RAM (which was not out at the time I ordered my RAM) as I do like the front independent, the tech, greater payload and everything being a bit larger in space in the cab and bed. I did have some vibration trouble with my GMC over 50 mph and there was a service update to the steering computer plus a road force balance corrected that. I did add some MODS to my GMC that have also made it even more enjoyable. Installed a Suspension MAXX steering stabilizer, Bilstein 5100s, Airlift airbags, and Retrax Pro Tonneau cover. Hope this helps thought you would be interested in some of my thoughts.