If you have Shimano shifters, then you need to buy & use Shimano derailleurs ...
Similarly, if you have SRAM shifters, then you need to buy & use SRAM derailleurs BECAUSE they are generally not interchangeable.
The derailleur cable "pull" rate of Shimano & SRAM shifters is different ...
FWIW. I have tentatively determined that you CAN "modify" a SRAM rear derailleur to possibly function with Shimano's shifters by excluding the dongle on the rear of many SRAM rear derailleurs ...
The so-called, fore mentioned exclusion can simply be done by routing the cable around the dongle OR by drilling a hole between the dongle & the derailleur.
The dongle is VERY EVIDENT on the cheaper SRAM rear derailleurs ... I don't know if that is the case with SRAM's better rear derailleurs.
As far as XT vs. XTR ... if weight isn't a factor, then consider a SHIMANO SLX rear derailleur, first. Otherwise, I would opt for an XT rear derailleur (with a "clutch") before the XTR rear derailleur (which has a huge difference in cost for less weight which is partially gained through its relatively-fragile-when-compared-to-aluminum carbon fiber cage; at least, that was the case in the past).