Any of the aftermarket boxes will net you some increase in power and fuel economy provided you can keep your foot out of the new found power. If you haven't already, you should check out the fueling box summary
that is maintained by Doug. Lots of good information there about the types of power adding and what you can expect in terms of power and fuel economy. Lots of folks are real pleased with the Edge EZ and the Van Aaken fuel pressure box.
2003 2500 Quad Cab, 4x4, ETH, 6 speed, 3.73 LSD, BFG 315/70/17, 4" exhaust, Scotty Air III