when you are saying crit build, do you mean mastery > crit > haste? i hope so.
play the specc you`re most familiar with. I tried affli a few times and always ended up better with destro - just because of the fact that destro is viable on every single fight and my hate against affli, I just got much more practice with destro the last few months.
how to play destro on thok?
normal rota until the interrupts are getting tricky / pala aura faded. then continue with felflame/conflagrate (during shout) -> cast -> felflame/conflagrate -> cast
play with havoc glyph and use the first one on the first guard + the second one on the bats for shadowburn
edit:
http://www.worldoflogs.com/rankings/...ction_Warlock/
here are the west ranks for Thok 25HC. you see that destro and affli are pulling about the same dps.