The '01 to '04.5 LB7 Duramax had injector problems. Otherwise they were good engines but I would avoid because of the expensive injectors. The '04.5 to '05 LLY Duramax was a fine engine, some report having over heating issues. This engine had the smallest fan of any Duramax. My '05 has 214,000 miles on it and doesn't overheat.
The '06 - '07.5 LBZ is considered the absolute best, basically flawless, highly sought after, usually expensive. The Allison transmission went from a 5 speed to a 6 speed in '06. Duramaxes from '07.5 and on were fine, no major problems, just gained emissions junk that hurts mileage and makes things complicated.
The statement that GM trucks carry less weight than the corresponding year Ford/Dodge trucks is simply not true. In '01 for example, a Dodge dually had a GVWR of 10,500 while GM was 11,400. The GM truck was also several hundred pounds lighter and thus had well over 1,000 pounds of additional payload.
I highly recommend a GM truck. I've had 4 of them and worked them hard in my business towing and hauling. I don't put up with machines that aren't reliable and work well. My GM trucks had extremely good track records. My current '05 dually has 214,000 miles on it. It carries my truck camper and family at about 1,700 pounds over it's GVWR. It rides and handles very well.