From a solo players perspective, haste is currently a better stat than mastery until really high gear levels; and then mastery overtakes it from haste. But they are really close together and based on your current levels of haste/mastery, it's very possible that simcraft would put mastery ahead for you, and the next upgrade haste would become better again. But they are not mutually exclusive, you need both. So going for a pure mastery build and avoid haste gear (or the other way around) is definitely not the way forward.

As a rule of thumb, with spell haste totem; 1423 & 2781 are the haste points where you gain an extra flame shock tick. That is w/o EM/bloodlust/dark intent, and for non goblins. (this is the spreadsheet that covers haste for flameshock ticks).

So basically favouring mastery over haste as a boxer ain't bad at all.