I personally feel Ford's 6R140 is much better spaced for Towing. This is also partially why a RAM with the Aisin AS69RC transmission will have more grunt at 1st gear with a 3.42 rear axle than with the 68RFE and 4.10 rear axle (the other reason is because it gets 65 more ft-lbs of torque). Here's the tale of the tape:
gear - ratio
Ford 6R140
1st - 3.97
2nd - 2.32
3rd - 1.52
4th - 1.15
5th - 0.86
6th - 0.67
Aisin
1st - 3.75
2nd - 2
3rd - 1.34
4th - 1
5th - 0.77
6th - 0.63
68RFE
1st - 3.23
2nd - 1.83
3rd - 1.41
4th - 1
5th - 0.81
6th - 0.62
So as you can see, the Aisin will get the load moving better for the first 2 shifts, but it's pretty even after that with the 68RFE possibly having better higher speed towing characteristics. The Ford has a better ratio setup in my book though. Much better step through the gears. I can see why they use a 3.31 rear gear versus 3.42. There was a good thread where a guy made some comparisons on maximum torque put down at 1st gear given rear ratios on cumminsforum.
