basicly its like
hit ---> around 25% crit -----> Spell DMG for Warlocks and Shadowpriests
hit ---> Crit or Spell DMG depending on your spec for mages
Hitrating no matter if its just +hit or spellhit should be your option to go for if you are a DPS class or a tank who is defense capped ... for example a warlock who is hit capped does around 34% more DPS on a 8 min bossfight ... rogues overall do around 40-45% of theyr dmg only with white dmg
more +hit is less resists more mana efficiency and overall more DMG wich means every yellow socket gets a +8 hit gem ... get the +15 spellhit rating for gloves (aint even expensive) etc.