I will respond based upon experience with the Duramax.
1. More GM dealers, with trained service techs, available throughout the country.
2. Maintenance items are less expensive.
3. Torque and horsepower bands are more efficient.
4. Initial purchase price average is $6000 less.
5. Duramax is quieter and smoother.
6. Loaded, or empty, the Duramax fuel economy is almost identical. The Sprinter is rated a little higher, but the Sprinter loaded MPG's seem to make the overall parameters nearly equal.
7. Handles better in windy conditions.
PS. IMHO, unless I had a guaranteed freight contract that required a taller load capacity, I couldn't justfy the expense. On the other hand, if I didn't really need the income, the Sprinter is more comfortable in which to live.