I love my '05 Cummins. Have had it for 10yrs and 90k miles (turned 150k today). If towing heavy, the auto is the wink link. The gear spacing is too far apart. A truck with the 6M makes a much better puller.
My dad had an 01.5 HO 6spd (3.55 gears) and it was comparable to my truck even with mine having the higher power output (and 3.73 gears). But I've managed just fine at up to18k gross.
The 03-04 trucks have a lower power output, and vary depending on auto or manual. The 04.5+ were all 325hp rated regardless of transmission. For the manuals, the early year trucks had the NV5600 6sps trans, and in 05 or 06 they went to the G56 (shorter OD ratio).
I'm going to get a little nerdy now.
03-04 autos had the ability to lock out 4th gear via the button on the shifter.
On the 04.5-05 trucks, the button enabled Tow/Haul Mode. This meant that the torque converter would now lock in 3rd gear, and lock earlier in 4th (OD). Also, manually selecting 2nd at above 25mph, the TC will lock (great for heavy towing in the mtn and keeping the trans cool). What you lost was the ability to lock out 4th. There is a dealer flash available to make the ecm lock out 4th at the loss of the T/H Mode (going to the older programming). There is also a way to add a switch on the dash to keep it from upshifting into 4th.
In 2006+, the new ECM had the ability to have a T/H Mode AND be able to lock out 4th.
2006 was the first year of the TIPM (Totally Integrated Power Module). Look it up and come to your own conclusion. It controls everything from turn signals to trailer lighting to the A/C Compressor clutch. If the TIPM senses a fault on that circuit, it shuts it off. Sounds neat right? Except if it happens too many times it needs to be replaced...
But, 06+ has that newer ECM that is EFILive tuning capable.
I love my Dodge/Cummins. Maintenance is easy, and with the age of it troubleshooting is easy as well (what little I've needed to do).
Would I own another one? No. I almost sold all to go buy a 7.3L gas Ford a couple months ago. On paper it looks like it would tow just as good if not better than my 16yo diesel. If mine got totaled, I'd be shopping for a new gasser based on my current towing load and frequency.