Get the 6.4. I have towed with a 5.7 and they will do the job but around here they have to be revved to the moon to make it up a hill. I had Cummins for about 20 years as well but I would rather poke my eye out with a stick than buy a newer model one. This one towed this trailer a few times up a 16 mile long 8% hill with 5500lbs inside the trailer. It made it fine but I felt like I was beating it. My nephew just paid a $1000 tow bill when his 3 year old dead stock Cummins had a trans (whats new?) problem.
I've bought the extended warranty so I won't be concerned with any problems like that for a while. If my truck breaks down in 5 years or 100k whichever comes first it's covered. Even the tow bill is covered. Heck I even get a free rental with the warranty I bought. The mileage difference alone is worth the difference between the hemi and the Cummins.
