I have a 2015 RAM 1500 5.7 hemi with the 8-speed auto and 3.21 gears.
It tows the load great. That 8-speed makes all the difference. It has a very low 1st gear, and so many choices in "overdrive" gears there is one for every load.
When empty (no trailer), it goes 70 mph around 1600 rpm and gets great gas mileage. When towing my 24ft enclosed trailer loaded to 8,000 lbs it works great.
On the highway it stays in 6th gear so there is enough RPM (~2,200) to have the right power.
I have towed 9,500 lbs a few times and in the flat east it does just fine.

So, based on that; get the 8-speed with 3.73 gears and you will be happy with both towing and driving without a trailer.

Now if you must tow 13,000 lbs, or mountainous areas; that might be another story.
I do find it odd you want to get rid of your diesel; unless you don't plan to tow with the pickup truck much anymore.