I haven't crunched math in a while, and college was a while back. This might be right, this might be wrong... I'll let others correct me.
First, you critical % and versatility % should be as close to eachother as possible:
(Second, I will be referring to crit and versatility 'effectively' increasing each other. To be clear, I don't mean the stat itself will increase, but rather the damage output, assuming 1% crit is an extra 1% damage)
If you're looking at things in a bubble- either critical damage or versatility, they work out to be the same. They do, however, multiply each other. In other words:
If you do 100dps, and you have 10% crit, you'll do 110dps. Adding another 10% crit will bring you up to 120dps because 100dps * 1.2 (20% crit) * 1.0 (0% versatility) yields 120dps. This makes sense, because 120dps is 20% more than 100dps.
However, if you're at 10% crit and add 10% versatility instead of 10% crit,
you'll boost your critical damage too. You're
effectively boosting that 10% crit to 11% crit which is a bigger boost. This makes sense once we write it out: 100dps * 1.1 (10% crit) * 1.1 (10% versatility) = 121dps.
121 is 1% (1% of your base 100dps that is) more than the 120 we got with 20% crit, because that 10% crit
effectively gained an extra 10% itself (Becoming 11% crit). That might look weird so here's the breakdowns:
100dps over 10 seconds with 20% crit (assuming 1.0 attack speed)
200 (crit)
200 (crit)
100
100
100
100
100
100
100
100
------
1200 damage / 10 seconds = 120dps (20% increase)
100dps over 10 seconds with 20% versatility (assuming 1.0 attack speed)
120
120
120
120
120
120
120
120
120
120
------
1200 damage / 10 seconds = 120dps (20% increase)
100dps over 10 seconds with 10% crit and 10% versatility (assuming 1.0 attack speed)
220 (crit)
110
110
110
110
110
110
110
110
110
--------
1210 damage / 10 seconds = 121dps (2
1% increase)
We can see that going from 20% versatility and 0% crit to 10% versatility and 10% crit is also a boost- the crit
effectively boosted that versatility to 11%, yielding a 21% increase total.
This continues to scale upwards:
30% versatility and 10% crit versus 20% versatility and 20% crit
100dps * 1.1 (10% crit) * 1.3 (30% versatility) = 143dps (43% increase)
100dps * 1.2 (20% crit) * 1.2 (20% versatility) = 144dps (44% increase)
And if we scale things up higher, and make it more lopsided, we'll see that the more crit we trade to versatility, the more dps we gain until we break even between the two:
100dps * 2.0 (100% crit) * 1.0 (0% versatility) = 200dps (100% increase)
100dps * 1.9 (90% crit) * 1.1 (10% versatility) = 209dps (109% increase)
100dps * 1.65 (65% crit) * 1.35 (35% versatility) = 222.75dps (122.75% increase)
100dps * 1.5 (50% crit) * 1.5 (50% versatility) = 225dps (125% increase)
Now for why versatility sims higher
First of all, clearly we want versatility and critical strike to be equal right?
Well, unfortunately, they don't scale the same. From what I've read on the
WoW forums, the crit rating and versatility rating are:
110 crit = 1% crit
130 versatility = 1% versatility
That means, assuming you have equal stats between the two, you will have a higher crit % than versatility.
Which increases versatility's value
In addition, the
raid buffs give more crit (5%) than versatility (3%), which means you'll actually want more versatility than crit before you're buffed (to bring them closer together).
Finally, if you have to
must pick between the two, keep in mind:
1) 1% versatility also grants 0.5% damage reduction
2) In PvP, crit damage is only boosted by 50% instead of 100%, meaning that 110 crit = 0.5% (essentially)
Making versatility hands down better in PvP-
20% versatility is better than 10% versatility and 5% crit (100dps * 1.0 * 1.2 = 120dps vs 100dps * 1.05 * 1.1 = 115.5dps) which means even without the extra damage reduction... when it comes to PvP...
stack the hell out of versatility
As for PvE, the values of crit and versatility are pretty close together. As you gain new gear, your crit and versatility levels will fluctuate, and whichever one is lower than the other will gain weight- this is why people constantly sim their characters. The more crit you have, the more you'll want versatility and vice versa.
If you
must choose between crit and versatility though,
you'll want versatility for the damage reduction. E.g, 20% versatility and 10% crit will yield the same damage as 10% versatility and 20% crit, but the former will give you a 10% damage reduction instead of 5%.