I definitely sided with the
Kematu. I usually go for the damsels in distress, like Saadia...but
I'm convinced she's lying and I didn't like that.
You guys already mentioned Kematu being more forthright, while Saadia seems evasive. I agree with all that, but it's really one person's word against another, and perhaps Kematu just has a higher Speech skill. I also saw other forums on this topic pointing out Hammerfall's history is more in line with what Kematu described, while trying to pickpocket Kematu shows he has official paperwork proving he's a legit agent. However, I think
Saadia gave herself away in her 1st conversation with you. It's like she tripped over her own lies, as I recall it was kind of like this:
Dragonborn: Hey, you know some people are looking for you?
Saadia: Oh no, I didn't know they are here, I totally can't believe it, I'm so worried I'm pulling a knife on you.
Dragonborn: Now, now, put that knife down and I'll try to help.
Saadia: Great! I know one of them is in the prison and you must interrogate him to find their location and kill them all.
I noticed how she changed her story. She appears surprised that the bounty hunters are around, yet she tells you exactly where to find one of them and then precisely what to do. That's a huge credibility issue and I was actually expecting a dialogue option to call her out on it. So even without knowing a lot of other information, it looks like Saadia's own lines give you a huge clue as to whether she or Kematu is more trustworthy.