I think this is a good point to keep in mind. It's almost the same case with talents - the new talents are supposed to be user-preference (it's not perfect, but it's the first run at the new system), but there are still people who claim that certain talents are "Take it or you're bad". There is no clear winner between haste and mastery. Pick one that is suitable for your raid team and don't let others weigh you down on your choice - until such time is determined that one stat is clearly better than the other.