My '02 Avalanche came stock with 13" rear, 12" front brakes (dual piston all around). When I did my full upgrade, I went:
- EBC Yellow Stuff Pads
- Summit Racing drilled/slotted rotors (not for performance reasons, more because they were a good price)
- Braided lines as the rubber lines were shot
- All new stainless brake lines as the stock ones burst
- GMT-900 front brake upgrade ('07+ GM truck brakes), which converted the front to 13" brakes
Most noticable was the hydroboost upgrade. After that it would be a toss up between the better pads and bigger front brakes. Since I did both of those at the same time it is hard to say but was very worth it. Truck used to brake like total crap and it now brakes really well.