Go with the spec that is ~1% higher on average
Go with the spec that is closer to the average every pull.
Crit you might get higher average number over a night of progress.
Mastery you'll be more consistent on your numbers.
Not saying one is better than the other, it's player choice
- - - Updated - - -
I don't argue or disagree with him at all, but let's say you go crit over mastery. You'll do crits let's say around 40% of the time if you go balls out on it, now you can be lucky one pull and get crits when it's optimal (adds, cds, whatever) or you can be unlucky. Same as our 4set, you could get lucky and chain proc 10 times when the Thok bats come out... or you won't. It's RNG and while on a complete night it's gonna average out to a number comparable to someone who went for mastery the person that went mastery is gonna have a pretty rough number that he can go on. I've tried crit build, didn't like it because it "swung" too much from the good to the bad. Does that make crit build bad or worse than mastery? No. Does it make mastery better than crit? No.
I'm missing a couple HC+WF pieces but I'm pretty much BiS if you ignore warforged, raidbuffed 17k haste and 20.8k mastery and 3.5k crit iirc. in full BiS (wf inc) I might go above 21k mastery and maybe 4k crit whilst keeping my 17k haste, I'm also draenei so I got a bit more room with the hit offpieces. Don't think I've seen anyone with higher mastery while keeping a reasonable haste number.