Originally Posted by
Yvaelle
The reason crit is often preferred by top spriests is because when you are going for a kill - mastery is pretty much useless. Mastery has no interaction with level 90 talents, Mind Blast, Shadow Word: Death, Devouring Plague's initial damage or Mindbender/Shadowfiend. So if you burst via Shadowfiend->Halo->Mind Blast->Devouring Plague->Procs / Deaths - none of that is really benefitting from Mastery, but all of it is benefiting from Crit - so the slim chance of getting a lucky crit somewhere in there is better than the 'nothing' Mastery is giving you.
With that said, when asked about this I usually explain all this - and then tell beginners to try Mastery rather than Crit. The reason is Mastery will feed you free procs more often, and it's more valuable for beginners to use what mana they have for Dispels - rather than worrying about going OOM all the time. Free procs give you something to do that doesn't cost you tens of thousands of mana a second - while keeping up your pressure - it also increases the mana return of VT via VT mastery procs.