I actually ended up calculating it (for all maxed characters), and there is near to no difference damage wise (both damage outputs are within 0.5% of each other). Without omegas phasma lead is slightly better. Once you can put omegas on poggle's leader ability it will clearly be the better choice. Even if you're level 78 or 79 and you can omega 86's special (-1 cd) poggle lead does slightly more damage than phasma.
As there is no difference in damage until level 80, you'll have to decide based on something else: Phasma lead gives you an additional chance to proc bonus effects that are caused by basics (evade for rey, ability block with poggle and defence down or advantage for phasma. offence up for gs is close to useless as you use poggle).
Poggle lead on the other hand gives you more control over the damage your attack will do. When I was using Phasma, there actually were some matches that i lost because i triggered a bonus attack at the wrong time. E.g. the bonus attack might cause RG to taunt when you were trying to put the toon in the 75-50% hp range to kill it without causing RG's taunt.
All things considered it might be best to use Phasma but switch to Poggle if your opponent uses RG or you can use omegas on his leader ability.