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Both haste and mastery have their strong points. Mastery seems to give you the most effective coverage at longer time scales, while they’re about the same at short time scales. Haste is arguably stronger (but not much) for 4-5 attacks, but at 3 or fewer it’s really a toss-up. If we’re assuming that we care about 5 or fewer hits, then you’re pretty safe choosing either as your go-to stat (after hit/exp caps, of course!). Again, haste has the advantage of giving you a non-trivial amount of DPS, which will probably tip the scales in favor of haste for hard-mode raiders. For tanks progressing at a slower pace, and thus not as concerned with personal DPS, exp/mastery is perfectly acceptable as well.
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