Once you have Unerring, Mastery gems should start to beat Int if you're above 520 ilvl. That's what I've been seeing, at least. If its close (Like .45 to Int's 1.00) go with gemming Mastery due to the undervalue of Mastery in Simcraft for Demonology. Sadly, they still haven't fixed it even though Zinnin wrote them a fix forever ago.