My experience, have had:
2000 Chevy 5.3 with 3.42 gears
2002 Chevy 6.0 with 3.73 rear
Current 2009 GMC 6.0 and 3.73 gear
2009 tows circles around the other 2 hands down, even going from the 02 with the older 4 speed to the newer 6 speed and higher hp/Tq numbers has made a real difference. Just my experience, and my opinion is the 6.0 with 6 speeds tows really well. If you want more performance with the newer trucks, look up Blackbear EFI live tuning. Yes this 600.00 service with custom tune to your specifications does make a difference...again my experience cause that's what I have done and very happy.