XT is more durable and basically bulletproof. XTR is for racers or posers. Look into Avid brakes, they are lighter and offer better braking power.

On my bike I use Avid brakes, Sram shifters, a LX front derailleur, a XT rear derailleur, a XT cassette and Avid 7 brakes. The LX front derailleur shifts just as well as the XT front.