Relentless Earthstorm Diamond Meta is 12 agility & 3% increased critical damage. The 3% increased crit dmg will apply to spells next patch, and I was wondering..

Relentless Earthstorm vs. Mystical Skyfire vs. any other caster meta gem. Im new to the forums so this may have been discussed before, but what is everyones opinion on using the 3% extra crit dmg over say, the Mystical Skyfire, or whatever meta you use? It seems it could be a decent choice for casters. Running with about 37-38% crit with fireball in raids, the extra damage could be a good amount. But I havent used the Mystical Skyfire yet, so I'm not sure how much it procs.