Haste = Mastery as far as I can see...Stacking either seems to give good results...I think a lot of it depends on your playstyle and if you're comfortable managing your mana levels.
If you stack haste you use Frost Armor, unless you can get ~27% raid buffed without it. If you stack mastery or can get high levels of haste without FA, use Mage Armor.
Exosphere - Mal'Ganis is an example of a haste stacking Arcane Mage who posts great logs. Check out his/her armory and WoL parses if you want to study that set-up. -- McBrain#1430
reply to cigtyme Make sure you use rune of power if you play arcane. Its sort of mandatory for arcane. If a fight is not feasible for rune of power then you are better off going frost.