If you are looking for speciality trucks like the C3 now Sierra Denali GMC only has that available, as far as drivetrain we share the exact same parts (LM7 LQ4 4L85-E 4L60-E NP246, etc etc), the front clips are about all thats different on the body (different front Fenders, Hood and Grill) I have a Chevy cause I was able to get an awesome deal on my truck (Fully loaded Z71 excab LS for $28,000 with all taxes included)
If I was to to get a new HD I would get a GMC only for 2 minor reason's, the bigger one being, GM I love their trucks and I like their V8 cars like the Vette, but I would NEVER own a non V8 GM car. This leads to the reason of GMC with not producing the cars Chevy does if you ever have to get service done for a recall or a defective part under warranty you don't have to sit in a line with them, when GM did a recall on the brakes on the new truck (Turned out it was just some stupid ABS warning STICKERS!!) the line to get into the Chevy dealership (Jerry Roth) was huge, Don Massey GMC was all oil changes..
The second reason on the HD's is I hate that new Chevy hood, it looks like a bug deflector is built into it hood.