Shimano XTR vs. XT Hydraulic Brakes
A very knowledgable bike builder gave me some interesting tips on the Shimano hydraulic brakes. Since the XTR components are designed to be as light as possible, the dual action controls hold less brake fluid, the cable/hoses are smaller ID, and the calipers have smaller pistons, which results in less braking performance. He suggested using the XT brakes if you are a very aggressive rider or a heavy rider and if you are doing a lot of downhill stuff because the XT brakes have a larger oil capacity and larger pistons which will produce more braking power.
I am not sure I would ever experience the difference in braking power since I am just getting back into biking and it will be some time before I am hitting the steeper slopes.