It's basically like well, the storm cloaks are a bunch of racist, and discriminative militia.
The imperials however are a real army however and they will be able to defend skyrim.
Negative point for the Storm cloaks: Ulfric doesn't care about the people, he only wants the power of the high king.
Plus, ulfric won't just talk to tullius, if he did he would find out that tullius soon wants to attack the thalmor.
The Last Dragonborn, if you're going by where the story line leads you, would side with Whiterun, and unless you push Balgruuf, he will stay Neutral.
The Last Dragonborn is above the selfish squabbling rebellion, becoming Harbinger of the Companions quickly, and once Alduin and Miraak are defeated, I like to imagine he makes a peace treaty with both sides, and leads the attack on the Thalmor with a host of Dragons, Blades, and an united army of Men.