Zeers or Chexmix in P3?

Troopers do better than Chex Mix in T6 and below because of deathmark, T6 and below the raid bosses have vast healthpool and Deathmark does about 70K per hit in t6. I used to run Zeers, Starck, Deathtrooper, Shoretrooper and Thrawn and my high score was 9,6 mil in T6.About 7mil of that score was from deathmark.
In heroic Traya has less than 1/3 of the health so deathmark doesn't do much dmg and she hits much harder and often so you can't get a better score this way.

Why not both? Zeers still works pretty well, just sub someone else in for DT.

A lot of the trooper teams seem to also include Thrawn, who is also a component of many p2 comps. So it depends on how you can split up your roster and whether your guild struggles or not.

The reason of the Chexmix is the ability to multi hit with zHan. For this reason, he's a far better character on himself all the mechanics of both raid and lead. From the lead, your Han reach 85% counterchance with the ability to hit 2 times in a row for the first counter. With the deathmark, it's not much about the damage you get from this debuff but it's for the effect of the "taunt" it triggers. From that point, with the Standalone and the offense up you'll gain something close to 15 000 offense or more depending on your optimisation.

With a zVeers, you get a multi assist attack and then what ? Nothing and you won't gain tenacity from anyone so that you'll get daze and stagger or debuffed.
zVeers is far better in p2 with a full trooper squad (no DT).