These calculations build upon themselves creating sound foundations (presuming they are accurate), not results that can be translated in to absolute certainty. (This is not to deter from the fact that fundamental, unequivocal numbers exist and can be recognized as such (i.e. the supplementary 17% to hit and stat scores.))
A clear example in which we can observe the effects of misinformation would be the case of the warlock-specific weapon enchants: Spellstone (60 Haste Rating, 1% DoT damage) versus Firestone (49 Crit Rating, 1% DD damage.)
When Ulduar was fresh, posts based on Simcraft and gear-optimizing programs were published. These determined the “exact” value of each stat and the public proceeded to treat these numbers as gospel. For Destruction Warlocks (0/13/58), it was ascertained that Spell Power was the most valuable, Haste Rating was its near-equivalent and Crit Rating paled against the two.
What people failed to understand while regurgitating these revelations was that these values were not universal; in fact, they were based upon very strict Best-in-Slot gear scenarios wherein Crit Rating was abundant compared to Haste Rating.
Soon after, every warlock began hoarding Haste Rating (so, using Spellstone), ignoring Crit Rating and the natural synergy that exists between the three stats.
To illustrate the aforementioned mathematical synergy of crit and haste one can look at a simplified equation:
Average Damage per Cast = x ( 1 + ( Crit / 100 )) + ( x ( 1 + ( Crit / 100 )) * Haste / 100 )
Where ‘x’ is the damage of your spell, unmodified.
What one can extrapolate from these variables is that haste is applied to the original spell damage after it has been multiplied by the critical chance (or vice-versa.) In other words, the two stats are applied multiplicatively and the more you have of one, the more valuable the other becomes.
So what can we infer from this in relation to our Stones? The more people continued to amass Haste the more potent Firestone became; yet, since Crit had been dubbed as unfavourable, it was dismissed altogether even though its second effect (1% to DD spells) buffed the sources of about 65% of your damage; a bonus which easily trumps that of the Spellstone.
Ultimately, which one is better? I can only tell you what is best for me. With my T8.5 and T9 I have an abundance of crit to the point where it's actually unpleasant; therefore, I use the Spellstone for its 1.8%~ Haste Rating. Without it, my incinerate would have a cast time of over 2 seconds which I think is unacceptable for a warlock my ilevel.
That is all for today. Join me next time for the following segment, "Pulling Aggro and Why It's Not That Bad Sometimes".
With love, Minina