If you go Shimano, I say go SLX M675 rear derailleur and XT shifters. That's what I chose, at least.
I would have agreed in the past, but for
me I'd run SLX for both. I've run XT shifters and the multiple-upshift kinda annoys me - I always seem to get multiples when I'm hitting bumps, exactly the wrong time to find yourself a gear or two too high. Plus I never have any problem firing off a couple presses when I do want multiple upshifts.
I got an all-SLX bike back in 2011, and figured I'd upgrade it as I went, but wound up really, really impressed with the SLX parts. If I was doing a budget build, I'd have zero problems going all SLX. For me, a higher level in the rear derailleur makes no difference, and I actually prefer the SLX for the shifters.
Opinions are like some body part, so take mine for what you will.