Figured I could give my 2 cents : I went haste>mastery and it ended up being pretty decent (especially since I used Healing Rain on CD during phase 2).
Here's the report (I'm Exo) but as you can see I play in a 25m guild so things might be different in 10m. I understand why you'd want to go for mastery>haste but it really falls down to whether or not you have the DPS to kill the orbs in time. If you do and even have some margin of error then go for haste, if you don't stick to mastery (although it's still random and doesn't boost HR).