Originally Posted by
voltaa
Going to have to disagree with you here, frost armor gives 7% (multiplicative) while mage armor gives 3000 mastery (10% flat)
While you may be thinking to yourself that 10% is better than 7% it really isn't since mage armor gives just a flat increase while frost armor stacks multiplicatively, giving you more stats than you would get normally from a flat boost.
Since haste and mastery are so close in value (regardless of what build you use) and the haste bonus from frost armor will likely put you over the 15 tick breakpoint for nether tempest, always use frost armor, the exception being later in the expansion when GCD capping during lust becomes an issue, at which point you would use mage armor