I'm a big cummins fan here but I also own a 6.2 with a turbo. You cummins guys need to give credit where credit is due.
Fact 1. The GM 350 diesel was the first production diesel in light duty trucks. Yes, it was a heap (although not a converted gas as everyone says).
Fact 2. The 6.2l was a huge improvement in every way to the 350. Power was anemic but mileage was great and that was the purpose of the engine in the first place.
Fact 3. The 6.2l caused Ford to bring the Navistar IDI v8's to market in their pickups. This spurred Dodge into looking for diesel power for their trucks (hence, the cummins).
Fact 4. Outpowered by the diesels of the early 90s, GM tweaked their 6.2 to add power through turbocharging and a computerized IP and the 6.5 was born. GM could have designed an entirely new powerplant but the 6.5 was capable to produce similar stock HP/TQ numbers of the early cummins (although the cummins was much more modification friendly and a more rugged design).
Fact 5. 6.5TDs still power humvees. With updated blocks with better metalurgy and improved fueling, The newest 6.5's are offered at 300hp to motivate the latest humvees with heavier armor designed to protect troops from IEDs.
IMHO, Cummins is still King, but i respect all diesels.