The 4r100 is no comparison to the 6psd trans when it comes to towing IMO.
I still have a 450ish hp 7.3 and the 6.7 will run away from it with a load. The tighter gears makes all the difference.
I love my 13 but as said before, get a 15+ if you can swing it.
The exhaust brake in the 15+ is a...