If you're using an engineering helm do you want to use a stat/hit gem, or two 600 stat gems.
Lets take warlock for example. My thought process has always been as follows.
The more hit you have on your gear, since it doesn't scale down, the more all of your other stats/items scale down.
By using 600 crit/600 mastery gems, you're going to have to reforge some haste or crit (weak two stats) into hit, which is going to scale down your overall mastery (your best stat by a lot).
By using 600 hit/600 mastery gem, you're going to leave some of the crit that you would have otherwise reforged to hit, but it's not going to scale down your mastery as much.
This logic seems sound to me, and I go hit/haste and hit/crit on my mage and moonkin, but I see so many people use two stat gems so I figure I'm wrong or missing something obvious. Thanks