Just past experience, but Chevrolet dealers have always been more willing to negotiate in my opinion as compared to the GMC dealers right next door.
I've been told before, "Well if you want to haggle go to Chevrolet, the price is the price here." This was at a GMC dealership.
I promptly went over to the Chevrolet dealer and bought an LTZ Duramax for the same price as the SLT Gas truck I was looking at.