I think people are blowing this haste vs mastery discussion way out of proportion and only looking at extremes. Instead of thinking "which stat do I stack" you should be thinking of which stat to avoid, i.e. crit.
Also ask yourself if you are 100% only playing afflic. I personally like to annoy people w/ resto lock spec in 2s and bgs so mastery is more valuable to me. Still too early in the game for multiple gear sets unless you have a fully functional gladiators sanctum. Let's take a closer look at the gear and you will see there's only a few pieces with options:
Helm: crit/mult vs mastery/mult
Neck: mastery/mult vs crit/haste
Shoulder: crit/mastery vs haste/mult
Cloak: crit/mastery vs haste/mastery
Chest: crit/haste vs haste/mastery
Wrist: crit/mastery vs crit/multi
Hands: haste/mastery vs crit/mastery
Waist: crit/haste vs haste/multi
Pants: mastery/mult vs crit/mult
Feet: haste/mastery vs crit/mult
I'm not looking at weapons, rings (most likely pve rings), or trinkets, that's already been covered elsewhere. I put the no brainers in bold and left the ones that give you an option for customization.
As you can see when you put our options side by side, the only real decision is between helm, neck, and shoulder. The other pieces there isn't really a debate if you value haste/mastery above all else. This is why you see people using pve helms (b/c the first boss drops haste/mastery) and this is why you see people break 4pc to stack haste (which I feel is not worth it for the marginal haste & mastery gain).
If I was a hardcore pve nerd killing mythic I would use pve wrist and rings and a haste/mastery belt if you can find one.
For head we have the primal gladiators cowl of cruelty (147 haste / 147 versa)
And pants we have primal gladiators trousers of cruelty (137 mast / 158 versa)
They require Ashran rep and also the other items like it are not worthFollowing stats priority like Chanimal said
, we should go for head and pants I said now + crit/haste neck + haste/mult shoulder
Helm: haste/versa (ashran rep needed)
Pants: mastery/versa (ashran rep needed)
Rings: haste/multi and mast/versa(legendary ring)
With haste enchants and 5% bonus haste from affliction, they give us (unbuffed and considering it's not 690,
cant do that in mrrobot.. idk why):
Share your toughts, please =)