You didn't mention X0 in your post. SRAM X0 is my favorite drivetrain, but is pretty comparable to Shimano XT.
Also, as someone mentioned above, you've gotta get one of the new clutch derailleurs. Recently had to replace a SRAM XX with a SRAM X0 Type 2 derailleur on a test rig and the Type 2 is amazing!! I don't think I've dropped a chain yet, and I'm riding a hardtail. VERY HIGHLY recommended.
Shimano does have their own version of a clutch, called Shadow Plus.
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