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.