Originally Posted by
Blackmorgrim
Actually when it comes to this, leaning towards crit or mastery (depending on your spec) all its really doing is making your damage less consistent, and increasing the variation (while slightly lowering the average). Leaning towards crit on something like engineers would only benefit you if you're relying on Mindspike or MB to kill it, otherwise, dropping a 2-3 orb DP and using insanity would continue to favor more of the balance.
However, splitting hairs aside, the difference between favoring crit and mastery is so minute overall. Going GCD cap/high mastery, vs DP3 and a balance of crit mastery is within the statistical margin of error... so as long as you have the 2pc bonus, really its only having at least the DP3 haste breakpoint that matters, after that, you can pretty much have stats however you like.
Otherwise, a simple example for the OP (assuming 1% crit = 1% mastery in overall benefit):
If I have 20% worth of stats to distribute, I could do:
20% crit = 1.2 X Damage
20% Mastery = 1.2 X Damage
10% Crit, 10% Mastery = 1.1 x 1.1 x Damage = 1.21 X Damage
Because crit and mastery have a multiplicative benefit, splitting between the two gives more of a benefit than dumping the pool into one side.