I say get phasma first, I see her more often than poggle in arena. She has two powerful abilities, victory march gives +50% turn meter and fusillade is an aoe with speed down, whereas poggle has the offense up and speed up with droids. I think in arena both can be used to equally devastating effect but in GW two abilities means Phasma is more likely to have an ability not on CD when you need it. Poggle's leader ability is only good with droids in your team whereas Phasma's leader ability is universally one of the best in arena/GW whoever is in your squad.
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Phasma is more versitile so get her first.