in my opinion it depends a bit on the fight... more movement makes fdcl stronger which favors crit. if you go better with mfi you'll want more mastery. its personal preferance also. so you can use a mix of crit and mastery if you really want to cover both..
i really can't imagine not going for heavy crit as long as you have 4 piece and/or amp. perhaps there is some amount of increased dps when going heavy haste when you never have to interrupt your insanity burns due to mechanics or SWD snipes on adds, just to pack in a few extra insanity mastery proc rolls, but the amount of offhealing you gain from amp w/ heavy crit chance is pretty significant - or at least that's been my experience thusfar.
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