Which one should I choose? Both claim to fight against the Aldmeri Dominion, but I personally think Kematu has a much better chance of defeating them than Saadia. On the other hand, what if I wrongfully convict someone? I can't believe most people can do this quest with no qualms. I mean, if Saadia was right all along and you deliver her to Kematu you might just give the Thalmor an edge because she's obviously a very important person, and I wouldn't wanna be responsible for that.
The mind is like a parachute: it only works when it's open
I normally choose her side but this last time, I chose Kematu and afterwards it never settled right with me. We can make our best educated guess as to who is lying and chances are, its Saadia, but Bethesda purposefully left out just enough details so that ultimately, it is up to the player.
There are a few reasons why I believe Kematu. First of all, he has no reason to lie. He could easily hire a merc (you) to find her or he could have decided to kill you with his six guards. Instead he decides to parlay. The only real thing that goes against him is that he's hanging inside a bandit cave, but that could be explained as just to keep from being detected by the Thalmor. The last thing that convinces me though is that he makes no attempt to kill her..only to capture her. Her fate will be decided back home.
To me, it's Kematu that's full of crap. First of all, these alik'r gang always refuse to actually explain what's going on, Kematu only offer his side of the story only after I had decimated his bandits. Yep I have reason to believe he's only making things up in last minute to turn me against Saadia after it becomes obvious to him that his money offers don't persuade me.
Well alright, Saadia too's not telling me much. So are we tied at this point?
Let's examine Kematu's story: even now Hammerfel's fighting the Thalmor. Good sense should say that Hammerfel would need every able bodied warriors and every resource to fight Thalmor, instead of squandering men and gold on a wild goose chase in skyrim. At this evidence alone I could say Kematu's claim that he's actually fighting Thalmor, is a lie. It's more correct to say that they are on the Thalmor's payroll, which explains their money, thus their ability to hire bandits, and thus this stupid but highly egotistical nature of this mission (seriously? All this trouble just to hunt down a traitor?). Or perhaps Saadia's wanted for some secrets the Thalmor wants? Who knows?
Besides, Saadia's not lying when she said I am her only hope. If she's actually an ally of Thalmor she would surely have fled to Summerset instead, or sought Thalmor protection. Kematu on the other hand? If his purpose is really to punish a war traitor who sold Hammerfel to Thalmor, then he should have been able to explain his case to Whiterun's Jarl, or perhaps seek Ulfric's aid. Nope. He refused to explain until he sees need to manipulate me instead.
Anyway, both Saadia and Kematu's not being truthful. But kematu's full of crap. That's my conclusion.
If Saadia was a big Nord male with a beard nobody would take "Saadia's" side. However, because she is rather sultry and "distracting" to say the least, they justify betraying Kematu with "self deceit" as someone else said ;P