Before buying my bike I rented bikes two different times on vacation. The first was a trek roscoe 7? and it had a lower level sram eagle groupset. The second was an orbea occam m10 with all shimano xt components. As far as use i didn't notice any discernable difference in shifting performance. The bike i bought has shimano deore shifter, crank, and derailleur with tektro brakes and don't see any real difference in it from the other two. Deore while shimano's lowest 12sp groupset is good but some components like crank and cassette are heavy. One cool thing about the shimano shifter is that the up shift lever ( to harder gear) can be pushed forward with the thumb or pulled backward with your finger to shift depending on preference. As far as i know sram is only thumb push for up and down. Another note for the shimano line is that the xt can do 2 upshifts in one push but deore and slx can only do 1. I will probably l upgrade the shifter to xt because there are many times i need to upshift quickly.
As far as brakes go i think they are a more polarizing subject than drive chain. Sram has had problems in the past with brakes causing many to steer clear. My LBS didn't speak highly of them but there are people who love them. Never really heard much bad about shimano. There are other good brake brands as well such as TRP and magura to name a few. Go for 4 pistons if you can. You can never have too much brake
Regardless of which brand you go with i would steer clear of the top of the line models (XTR, XX1) unless you just have money to burn. They are super nice and light but they are expensive and usually their light weight parts also wear faster. Shimano SLX and XT and Sram GX eagle are all good, reliable components. If you are on a tight budget dont be afraid to go shimano deore. Its a soild group set as is but with a little mix and matching (xt shifter and slx crank) you get a really nice groupset.