Utility
The GMC's more sizeable trunk will accommodate more large things than the Chevrolet Silverado 1500.
The Silverado 1500 and the GMC Sierra 3500HD can accommodate the same number of passengers.
The GMC Sierra 3500HD is likely a better choice than the Chevrolet Silverado 1500 in terms of transporting cargo, especially if you often need to carry a lot of equipment or massive pets.
Towing is considerably more the GMC Sierra 3500HD's forte than it is the Chevrolet Silverado 1500's.
Performance
With that kind of horsepower, both the Chevrolet Silverado 1500 and the GMC Sierra 3500HD are genuine speedsters.
Dimensions
The GMC Sierra 3500HD is tremendously more bulky than the Chevrolet Silverado 1500.
Your wallet will hurt more every time you fill up the GMC Sierra 3500HD than the Silverado 1500 because of its larger tank.
The Chevrolet Silverado 1500 and the GMC Sierra 3500HD compete for the same parking spaces.
The Chevrolet Silverado 1500's shorter wheelbase will likely give a more maneuverable feel in daily traffic. On the flip side, the Sierra 3500HD is likely to have a smoother, more stable ride.
Cost
The GMC is considerably more expensive than the Chevrolet Silverado 1500 in terms of MSRP.
Handling
With its tighter turning circle, the Chevrolet is definitely more agile than the GMC, something to consider if you do a lot of city driving; however, remember that a vehicle with a tight turning circle may feel a bit twitchy on the highway.
Drivetrain
The GMC Sierra 3500HD's engine is substantially larger than the Chevrolet Silverado 1500's. Keep in mind that bigger engines may use more gasoline than smaller ones.
Comfort
While the front cabin in the Chevrolet offers a bit more head room than the GMC Sierra 3500HD, there frankly isn't much of a difference.
Warranty
The Sierra 3500HD and the Chevrolet have the same basic warranty.