Okay, here's the facts: Whiterun kicked out Kematu's boys and imprisoned one of their members. In addition one of the bandits at the beginning of the cave say he doesn't like the Alik'r around, while his friend says something like don't complain unless you want a knife in the back. That's pretty compelling evidence against them in my book.
Then, after you slaughter them, Kematu tries his hand at smooth talking, and constantly calls Saadia a liar. Anyone that bases their point solely on another person being a liar, well that's just very dodgy straight off the bat.
As Slade said here, her urn is in Whiterun, if you side with Kematu, meaning that while she might be taken back alive, she gets an execution at some point. And as stated, the Thalmor don't have the right to just kidnap people in Skyrim, they would have to hire someone else to capture her. This is why I believe Saadia.
So to break it down, Kematu's Alik'r are mercenaries that made 2 groups feel threatened, they were most likely hired by the Thalmor to capture Saadia and bring her back, and Kematu lies to you out of desperation in the cave, after you've destroyed his forces. Side with the Alik'r and Saadia will be executed.
I'm going with Saadia, she's not hanging out in a damn cave, even that is pretty friggin suspicious!