Revenge, Kill Francis, Save Ivan. I'm pretty sure anyways.
Niko was... somewhat... a man of principals. Nothing in his nature seems to say he'd let it slide that this guy betrayed him and put Roman in danger just for money. He's in Liberty City to hunt down a guy who done almost exactly that.
I feel like he'd kill Francis because he admitted he's got dirt on him. This is sort of a tough choice though, because Derrick seems like a waster and I think Niko might be thinking of putting him out of his misery. I still think it'd be Francis to bite the bullet though.
And Ivan was set up and Niko doesn't like Vlad, so I feel like he'd let him go more on that basis alone.
Another big choice is Playboy or Dwayne. I think he'd kill Playboy. If you watch the cutscenes and listen to him talk, it's obvious why.
Some small characters like Cherise, I think he'd probably spare her. He wasn't asked to kill her, so he'd probably ignore it. And Clarance I think he'd kill because that's the job he is being paid to do. I don't think he had any massive quarrels about screwing over Francis at this point so he'd probably just do the job.
Darko is one I just have no idea. I feel like in the way it was written that they wanted you to spare him. I think MAYBE that'd be the choice Niko would go with? But it's a longshot. He'd kill Dimitri I'm sure of it, but why not Darko?
I think that's all the choices in the game anyways.