Haste outweighs mastery by far, until you reach 30.1% haste, or to be more precise, these exact haste rating values:
|With dark intent
||Without dark intent
These add the 2nd extra tick on your Vamp Touch
After that is when haste and mastery become on par stats, meaning that on single target fights they yield the exact same results per rating.
Which you should reforge for depends mostly on the fight mechanics, having fights with burst phases, like Hagara and Spine favor heavy mastery builds, but most other fights favor heavy haste builds. Morchok and Warmaster 'cause of multidotting, Yor'Sahj and Madness 'cause of AoE.
Ultraxion is neutral 'cause of the thing mentioned above (both being on par on single target), but you should also favor haste here, just in case RNG doesn't serve you well, and you don't get to clip your AA/SF in-between the 2 times raid members get fading lights.