For a 2500, the 8 speed starting in '19 has a much lower first gear and tighter split in ratios in the first 5 gears. Basically, on the 6 speed you have first gear through direct drive (1:1) is 4 gears, while on the 8 speed first through direct drive (1:1) is 6 gears. On the 8 speed, 2nd gear is about the same as 1st gear on the 6 speed. The combination of the lower 1st gear and tighter splits makes the truck feel peppier/faster on take off. However the highway cruising gears are almost identical. This means the top 3 gears on both trans, which are the gears you will be in when cruising down the road at highway speeds. On a 6 speed that would be 4th, 5th, and 6th which are almost identical ratios to 6th, 7th, 8th on an 8 speed. With an empty truck it's unlikely you would shift down more than that unless maybe climbing a mountain pass.
I was shopping for a new truck when the 19's with the 8 speed became available and test drove several. I did wind up buying a '16 with a 6-speed because it only had 24k on the odometer and was $20k cheaper. Yes, the 8 speed made the newer models feel stronger during acceleration from a stop (exact same engine and gear ratio) but I'm cheap and decided the 8 speed wasn't worth $20k. Does the 8 speed shift better....from test driving I can't say it was noticeably better. Maybe if I drove one more now I will say that. Some people claim it tows a lot better. From a stop it should feel better, but on paper there isn't any real reason it would feel stronger at highway speeds because the top 3 gear ratios are the same. But have obviously never towed with an 8 speed truck.
More reliable? I mean, we will see. Maybe they are? Everybody claims that but keep in mind that the 8 speed 2500 trucks are "only" 5 years old meaning there is simply not a lot of older high mileage trucks running around. The national average annual mileage on a pickup is under 12,000 miles. This means the average '19 2500 only has 60,000 miles on it. On the other hand I think the 66RFE was introduced around 2013 in the 2500 truck? So you are talking 11 years at an average 132,000 miles, or over the double the mileage. Of course you would see more failures reported on an older truck! My last comment is that failures or issues are blown way out of proportion on the internet. That is because anybody with an issue will jump on every Ram related forum they can find and complain their transmission failed. However nobody just randomly makes a post saying "hey, just wanted to let you know my trans has not failed....". Again, maybe the 6 speed failure rate is higher than typical but you can't confirm that based on browsing the internet. Let's say 1/2% of Ram 2500 6 speed trucks have a trans failure. Ram sells almost 500,000 Ram 2500's annually (not sure exactly how many are 66RFE trucks), but let's say 200,000 of them are. Even though only 1/2% have problems, that is still 1,000 trucks with issues. Now each person makes 3-4 different posts on different forums and Facebook groups and seems like a LOT of trucks have issues....even though it's only 1/2% of the total. Then on top of that, anytime somebody asks the question about the 66RFE you get 20 people reply back that they are junk and will explode and kill a bus full of nuns. Not because they personally have had a trans failure, but simply because they read a post 3 months ago about a guy who did have a trans failure.