Actually the GM 6.2 Diesels last a really long time. My Dad has a Suburban (Among four of five other other vehicles) with one in it that got about a million miles on it. (We're not sure exactly, he's rolled the odometer twice, and it was around 400,000 miles when he got it.)

All his 6.2's run great.

But they have crap for power and acceleration. (94 stock HP, I think. It just feels fast after driving a GM 350 diesel )

Also, they don't start in the cold without being plugged in. (Plug them in and they strart fine though.)

I'd say go for the Cummins. My 1st Gen Dodge has 283,000 miles on it and it still starts on the first crank, and will out-accelerate any of Dad's 6.2's (the one with 4.10 gears comes close though... mine has the 3.08)

The guy I got it from took good care of it. He says it starts fine in the cold, but he's always kept it plugged in to go easier on the engine. (I haven't had it long enough to find out yet.)