After the Settling Kadara questline has been placed on hold following Night on the Town, go anywhere else in the Cluster, then check your email for a message from Sloane Kelly.
Return to Kadara and meet with Sloane in the Outcast HQ.
The conversation will go a little differently depending on your previous interactions with Sloane, but they end the same way, with Ryder joining her for a final showdown with the Charlatan. Drive out to the navpoint she gives you. Don't worry about your weapons and squad- they won't be a factor.
Once inside, the identity of the Charlatan is at last revealed. Spoiler, technically.
He challenges Sloane to a duel, but Ryder spots a sniper deeper in the cave. You have a quicktime event to save Sloane's life.
Ignoring this prompt is the only way to remove Sloane from power in Kadara. If you do choose to save her, you'll have another quicktime prompt to shoot Reyes as he flees.
This choice doesn't matter much- even if you take the shot, it's a minor wound to the shoulder. Either way, he gets aboard a ship and lives to fight another day. As you exit the cave, you can start telling Sloane how things are going to be in Kadara now. Visit her again in HQ to negotiate the establishment of an outpost, and the way forward.
If you let Sloane die, you have a tense conversation with Reyes. If you're currently in a romantic relationship, you can choose to sever it now, or continue.
Whichever you decide, meet with Reyes again in Tartarus to iron out the future direction of Kadara.
You can choose to part as lovers, partners, or frosty allies of convenience, though Reyes will at least act friendly toward you no matter your choice. You may also want to drop in on the 'official' ruler of Kadara- Neema Dohrgun
She'll shed more light on the current situation as it stands. If you want to be reminded of your sins, you can visit Kaetus in the cells, who knows the role you played in Sloane's death, and vows to kill you if he gets the chance
So, who to choose? In this game at least, there are no practical reasons to choose one over the other. Both will allow you to establish an outpost, and neither gives you better numbers, or special bonuses between now and the endgame.
Metaplotwise, you might lean toward Reyes. He can't die, and seems like a strong candidate for involvement in a DLC or sequel.
In terms of the story itself- tricky. Reyes seems like the stronger choice. His Angaran figurehead is good for local and Nexus/Aya relations (you recall Kadara was Akksul's example on the danger of trusting humans). And while Sloane was the embodiment of the mutiny, the Charlatan and his people will probably find it easier to work with the Nexus, and the outpost. He also seems less inclined toward sheer brutality than Sloane. On the other hand, Sloane does warm up to Ryder after you save her life, and indicates a new path for Kadara. And for all her coarseness, she's never lied to you, betrayed you, or manipulated you. She also trusts you, as much as she trusts anyone.
The last key determinator is that Reyes kills Sloane in the most dishonorable way possible. Just shooting her point-blank in the face when she walked into the cave would have been nobler than luring her into a fake duel. Even if your Ryder can stomach that sort of perfidy, who knows when some hidden sniper rifle will one day swing toward you?
TL:DR - there's no game bonuses for either choice, take the path you find most interesting.