As far as I know, haste makes your dots tick faster, gargoyle cast faster, pet attack faster, and a better chance of getting free death coils.
Crit just gives you a bette chance to hit for double damage.
What should i stack?
it's actually mastery for a couple of reasons. the main one being that haste does not much for you if you are kiting or being kitet. if you feel that you have close to 100% uptime on your targets, it would be haste (for me that is).
anytime else, mastery all the way.
/edit: the main reason for haste in pvp is indeed the decreased rune-reg and not more dot-ticks. crit for unholy is mainly worthless, since our damage comes from so many different sources at once and it's more about utilising your necrotic stacks with a healerlockout when you ghoul is up. that ghoul is a huge damage gain.