for heavy duty Towing I think you'll find a DT-466 / DT-466E has good brute force low-end torque.. I dont know enough about the cummins to comment..
but to me the V-8 Diesels dont have the low-end that you really want for towing... more of their power is up high.. which is Ok for a kickdown to pass someone on the freeway if need-be..
I felt like my DT-360 Bus coming off a dead stop has more brute torque than my new bus wioth the T-444E... the DT-360 backs off earlier in terms of seat-of-the-pants torque than the T-444E but I think yopu would like a DT better for towing..
someone else can chime in about the Cummins in terms of that...