First of all, I'm sorry for yet another Stormcloaks vs Imperials discussion. However, this one is different. I know everyone reasons about the civil war using their own arguments.

In my current playthrough I'm trying to choose a side based on only in-game information. I have spoken to Ulfric and Tullius and their close friends. I haven't found any books (yet) that explain why I should prefer one side over the other. I'm looking for hints on the NPCs I should approach who can give me information about why I should choose the one side or the other. Anyone who has done this before?

I think it's probably the smart way to do it.

I've been spoiled about the outcomes of the civil war which has skewed which side I ended up going for. While previously I was for the Stormcloaks initially, but I disliked the outcome of their victory despite believing in their cause.

Of course, my in-game character wouldn't have made those decisions knowing what will happening, but what he hoped would come about.

So I'd say your method of choosing sides is probably better than mine where I had to retroactively rationalise my character's decision.

There is a difference between the meta-discussion of the subject that often takes place on this site and what I consider to largely be the "role playing" perspective, where you make the decision based on what your character would know and how they would evaluate the information available. The former is useful for discussing the topic, since it allows you to address as many of the pros and cons as possible. The latter is more applicable to playing the game itself.

Besides Ralof, Hadvar and their families, you have Ulfric and Tullius, their men, Sybille stentor, Vignar Greymane, Thongvor Silverblood, Dengeir, Skald, Torygg,...