The SRAM X.9 rear mech loses the carbon fibre and titanium of the X.0, but nothing in terms of function.
The extra weight is balanced by less fragility, too. It’s still as snappy in terms of changing, though, with the same 1:1 dirt cable tolerant leverage and single tensioning spring which stops it clattering off frames and generally keeps it more stable over rougher ground.
The straight line cable routing gives a low, bush-proof profile, and it comes in short, medium and long cage options. Make sure you’ve enough spare chain to cope with suspension pull, though, because the results can be cataclysmic at full travel if you haven’t.
The jockey wheels wear out faster than with Shimano, too, and the whole construction tends to snap rather than retrievably bend if crashed hard. It’s £20 more than same-weight XT.