Yeah this is a question you wouldn't ask if you ran high keys. Not that you can't enjoy WoW, be a wonderful person, and never run high keys. But yeah, M+ is very much about being able to kill trash fast, because on high keys some trash packs are both dangerous and time consuming.
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The heart of my original question is whether there are some instances where it is clearly better to play one or the other. I addressed this thread to those who play both, because I'm not about judging people's preferences.
As tank for example, I try to keep multiple tank specs ready to go, because some instances are easier to tank as BrM, some as DK, some as Pally, some as DH... BrM is my current favorite, just because I find it most fun atm, but I like to think of them as golf clubs in my bag - different clubs for different situations.