SRAM ESP derailer systems use a 1:1 actuation ratio, ie 1mm of cable pull at the shifter gives 1mm of movement at the rear mech. Shi**no use a 2:1 actuation ratio; 1mm of cable pull gives 2mm of movement at the rear mech. The SRAM system is claimed to be less prone to going out of adjustment, cable gungification and requires less effort at the lever.
SRAM do make a Shi**no compatible (2:1 actuation) version though called Rocket/ Attack so check the packaging before you go buying a new mech. If it's marked ESP it's not compatible with your XT mech.