At first I though Saadia was right, because as Tae-Rai said, basic Damsel logic. But I began to doubt her story, so I decided to see what happens when I go against her. I found that Kematu was really nice and that it was more likely that Iman, or whatever her name is, was in fact allied with the Thalmor. So I ratted her out and let them capture her, and look who comes knocking on my door a few days later. A few pointy-eared bastards face down in a pool of blood was enough to convince me that Kematu was in the right.
First play through, I sided with Saadia I think, simply because I wasn't accustomed to the open nature of quests in Skyrim and I thought it'd be a simple "Go to X, kill Y" type of quest I was used to.
Ever since, I've sided with Kematu. I didn't quite trust her the first round, and the more I think on it, the more I'm convinced Kematu is correct.
Ignoring the meta knowledge that Hammerfall fought against the Dominion actively during the war and the Redguards don't support them (thus Saadia is almost certainly lying), why would you send 7+ assassins to kill one fleeing, broke noble in hiding? That many armed reguards would draw attention from local authorities, especially when they start harassing women throughout the holds. What sort of assassin would announce their intentions and locations like that?
A large group of well-trained warriors with distinctive clothing and weapons, scouring the province for hints of one woman makes a lot more sense to this blogger if they're not worried about getting caught because their bounty hunters looking to secure a wanted criminal from their homeland.
A couple of things, After the completion of both sides Saadia's side really leaves you feeling manipulated. Well at least it did me, given the fact that I had previously read about the Great War and Hammerfell's departure from the empire. I had already known that the Aldemeri Dominion had taken allot from the Redgaurds in Hammerfell. Kematu's dialogue at the end of completing his side of things was kind of like the nail in the coffin for me. I always side with Kematu now.
The penalty for that is worse if you're military. Specifically, if a military member commits treason by giving away any kind of classified information to a foreign body (government, ISIS, etc.), it's classified as espionage under the UCMJ. The penalty for espionage is up to and including the possibility of death.