Hi,
Just had an accident and broke my rear derailleur and its hanger. (Besides my left elbow and some other injuries I got in in my knees, arm and face)
My bike configuration is : SRAM X7 shifters and SRAM X7 rear derailleur.
Since I need to replace the derailleur, I was thinking to upgrade it to X9; my question is: does it make sense? I mean will I really notice the change (cost/benefit)?
A friend told me to install a shimano rear derailleur instead -either SLX or XT- of SRAM. Do yo know if those are compatible with my shifters? If so, should I stick with SRAM or change to Shimano?
I will not be able to ride my bike couple of months... But would like to receive your opinions the soonest.

I'm assuming you have regular SRAM triggers and not SRAM attack. You'll need to stick with SRAM since the pull ratio is different for Shimano (1:1 vs 2:1). You won't see much of a different between X.7 and X.9. Very small weight difference, and it might stay in-tune a tiny bit longer. I don't know what the price difference is, but you can decide for yourself whether it's worth it.

SRAM vs Shimano is a personal preference thing. X7/X9 and SLX/XT are all very solid components. You can't mix/match SRAM and Shimano for the rear derailleur/shifter -- they have different pull ratios. You can mix SRAM/Shimano on the front derailleur/shifter.

You're not going to notice much(if any) difference in performance going from a standard x7 to standard x9. But, if you happen to have a 10sp drivetrain, you would notice a difference in the new style SRAM type2 rear derailleurs. Type2(or shadow plus for Shimano) has a mechanism that prevents the cage from moving forward which reduces chain slap and provides better chain retention. Something to look into if you have 10sp.

Upgrading shifters make more of a difference than the derailleurs. My "good" bike has x7 rear but x0 gripshifts for instance.

^^ Less of a difference with triggers. Paddle shape and ergonomics is about the only difference I've found.

Thanks a lot for your advices.
I have 3x10 drivetrain... I will try to find SRAM type2.

Just make sure to get the long cage version for your 3x10. SRAM makes type2 in the x7 and x9 models.

Type 2 is the best thing to happen for an RD - not only for the clutch in chain retention, but the ability to lock open the RD for rear wheel removal and chain removal. Very convenient when you don't have a third hand. That feature alone is why I am never going back!