I have the 6.4 with the 6 speed. The issue with the 6 speed as mentioned is it has the same gear ratios as the 6 speed behind the cummins (66RFE vs 68RFE). That said, the gearing in first is a 3.23 and 2nd is a 1.83. That hurts the towing with a heavy load behind you. Going lower it makes a huge difference without much loss on top end. As Toddz mentioned above, it brings our hemi trucks closer to the ford 6.2 with the 4.30 rear gears.
For comparison sake, the Ford 6 speed has a 1st gear ratio of 4.17 and 2nd gear of 2.34. The Ford tranny is geared alot lower in first and a bit lower in second compared to our Rams. I only run about 2k rpm in overdrive (6th) on my truck running 70 mph. In 5th when towing I'm about 2500-2600 rpm. And I'm running 34" tires, half inch above the factory.
I would seriously consider the 5.13 if I were doing it again. But my installer was super impressed with my truck after he did the swap and he drives a 2019 Cummins 2500.