Kematu. There are rather glaring holes in Saadia's story if you know the backstory.
Namely... that a group as hardcoe Redguard as the Alik'r would not be Thalmor lackeys. Combine that with the Alik'r staying their hand after you cut a bloody swath through the bandits between them and the cave entrance, and how Kematu only sacks Saadia instead of killing her (and by extension you as a witness to the event) and you have your answer as to who's telling the truth.
This, basically.
both sides lie to you about the facts. it all comes down to who you want to believe.
The fact that the hammerfell men are bothering women all over the place and are not even allowed in cities due to their actions is all the proof I need.
I ALWAYS side with Saadia. Everyone deserves a second chance. Since most of my characters are all about second chances, they feel for her. And sometimes it has to do with the pretty face.
If Kematu was legit, why are his men in prison? why is he hiding in a cave with paid bandits to keep you out EVEN if your there to help him? Only when he realizes you killed all the bandits does he try to pay you to change your mind? My characters don't need to pay to complete a task and paying them to change their mind NEVER works.
That's silly.
I mean, I understand the whole, "everybody deserves a second chance" thing, but I don't really think it applies to Saadia. She holds you up with a knife and lies to your face right out of the gate, and clearly doesn't have any remorse for her betrayal in Hammerfell. While the Alik'r are clearly a bit rough, that has more to do with the fact that they're a no-nonesense band of elite warriors then any kind of maliciousness.
Let's be frank here, if Saadia were a guy practically every Dragonborn that did this quest would turn her in at a moments notice. Her gender is basically the only thing that lends her story any weight.