I always preffered X9 to XT kit, but am swinging the other way for 3 reasons:
Having to disassemble the X9 shifter to change the mech cable. seriously wtf? being the sort that dives in and doesn't read the instructions means that this killed the shifter as all the springy bits escaped. Its such a common procedure to have to do on a bike it seems ludicrous to make it intentionally harder.
X9 shifters have started sticking on me on the downshift. Changing the shifter helps sometimes, changing the mech has no effect, changing the gear outer had no effect. I got stuck in 3rd gear coming out of the gate at a DH race. I nearly threw my bike through a caravan.
The newer Shadow Shimano stuff has the top entry cable routing, and two way release, fixing my only 2 issues with Shimano kit.