General rule about that 13 163 rating breakpoint i talk so much is to go for it if u can get it without geming/enchanting haste. Only with reforges.
As arcane mage mastery rating will be always you best secondary stat.
Now about the armors - for single target go with frost armor and for multitarget (understand fights where there are atleast 2 targets in NT+ArBarr cleave range) go with mage armor. In your case i can only recommend you to follow that rule. There was recent advise here in this forum that the closer you are to that 13k haste break point the more you can forget about armor dancing and go all out mage armor for any fight.