This should be pretty easy to figure out with some basic math, but it depends on the rest of your gear and your values for CC, CD and Elite % outside of the socket. Short answer though, Diamond is very underpowered and the Emerald will almost always outperform it.
Assuming you have say, 50% CC and 350% CD not counting your weapon (50% base, 50% paragon, 100% amulet, 50% ring x2, 50% gloves). Your current crit multiplier is:
1 + (0.5 * 3.5) = 2.75x
If you add 130% CD from an emerald your CD goes up to 480% and your new formula is:
1 + (0.5 * 4.8) = 3.40x
This means your effective damage increase is 3.4 / 2.75 = 1.236 or ~23.6%. Right away it should be obvious that this is going to outperform a diamond since diamond only gives +20% vs Elite. But let's continue anyway. Let's say your damage vs elite right now is 45% from Aughild's 3-pc and SoJ. That means your elite damage multiplier is 1.45x. If you add a diamond, that will increase to 1.65x, which gives you an effective damage increase of 1.65 / 1.45 = 1.138 or ~13.8%. If you're also using a high Elite % Sunkeeper (say 25%), then your increase from a diamond will only be 1.9 / 1.7 = 1.118 or ~11.8%.
tl;dr: The emerald will always outperform on a well geared character, and can even be up to twice as effective as a diamond (or more).
