Stormcloaks. Ulfric Stormcloak is the true high king of Skyrim he won far and square. Plus Imperials seem to have no problem just letting the Thalmor ride them like a lady of the night. Its been 200 years since the Oblivion crisis they need to get their s*** together.
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If your basing this off how "fun" it is. Then both sides are similar, but not when it comes to Whiterun. And imo the Stormcloaks will provide more excitement.
Imperials: Organized, Discipline, Respectful. But, they also don't understand Skyrim and it's ancient traditions. And General Tullius doesn't care about you being dragonborn or anything with the greybeards as much as Ulfric does.
Stormcloaks: Tradition, Religion, Honor. But, when compared to the legion they seem more barbaric. Which is mostly because they are a rebellion. And no the Stormcloaks aren't racist for the most part. The city that is their stronghold has citizens in it that are racist. Should Ulfric help the dark elf refugees and allow the argonians into the city knowing it will cause trouble? That is something you would want to consider if somebody says he is racist. And Ulfric does show some power hungry qualities, but he is honorable.
I have done both sides and I prefer the Stormcloaks. But, in the end it is up to you. Only pay attention to the posts that aren't one sided. I know in this one I favored the Stormcloaks, but that is because they have a lot more to put an opinion on. This is all my opinion. Hope it helps.
"M'aiq has heard legends of a man with a powerful beard. This man never sleeps, he only waits."- IamI3rian