Haste and mastery tend to fluctuate as to their value regarding each other. It's not a case of get x amout of haste then go mastery.You will need to sim yourself and see which is currently better for you, though even then you might find that due to how variable the stat weights are the sims may so haste mroe valuable so you reforege to haste over mastery and then sim yourself again and it will return Mastery is more valuable than haste.
At the end of the day Both stats pale in comparison to strength and crit, so take whatever piece allows you to maximise those two stats.
Thanks, Teng. I am planning on trying out a haste heavy arm spec for a day or so just to see how it feels. I gemmed the new helm I linked, and restored everything prior to doing anything. Being over the hit and exp caps (7.5%) right off the bat is nice, and I am also nearly doubled on the haste value. Now I plan to mess with more haste, maybe try to get the swing speed down to 2.9 seconds or less...
For me mastery>haste but it's pretty close and I expect haste to gain some more value with RPPM trinkets and the legendary meta. As Tengen said simcraft is pretty nice just import your character and get values for all the stats you want.
For me haste(0.41) and mastery(0.43) are so close that it doesn't really matter if the helm has haste.