There are positive things to be said about SRAM's shifters/derailleurs, but Shimano has a greater variety of derailleurs which are designed for different riding conditions ... the SAINT components, for example, are much more robust (and consequently, heavier) than their "regular" MTB stuff.
The new, low profile Shimano "Shadow" design is (IMO) a great innovation in rear derailleur design, and would be a benefit to many cross country riders.
So, if you want-or-value continuity between your various MTB bike's components, then you should probably choose Shimano.
If you are a sponsored rider, then use what they give you.
I went from X-9 back to XT on my bike this year and I had to learn to pause while shifting all over again. The X-9 shifting is so solid and so much quicker I did not realize it until I went back to XT.