I had a '14 Ram 2500 with the 5.7l. Apart from numerous problems, I hated driving and towing with it. The transmission mated with the 3.73s was terrible. It was always in the wrong gear and would get bogged down. I ended up trading it for a new 6.7 CTD.
The 6.4 has slightly more power, gets better mpg because of the cylinder deactivation (5.7 doesn't have it) and has 800+lbs more payload. If you want gas, go with the 6.4. I'm not sure if they share the same tranny but I have heard the same complaints for both.