Stormcloaks- Pro: Honorable, standing up to their ways, their traditions, what the Nord stands for and the Nordic way of life.
Con: Stereotypically, the stormcloaks are racist, but that's just Ulfric and many Nords and dunmer are racist to begin with. Ever been to Morrowind? If you were any other race but dunmer, you are hated. If you're a dunmer, you're an outsider. Dunmer are getting what was coming to them in Skyrim.
Imperial- Pro: They've protected all of Tamriel with the divine wisdom of the Septim bloodline. Now that it's shattered, it's slowly falling apart. They only want to keep order and peace across Tamriel.
Con: They can be seen as an evil empire. They're loosing power over Tamriel and will stop at nothing to lose it. A "peace treaty" and "surrender" is very much the same. Final Fantasy XII anyone?

The problem is, is that if you're not a nord you have NO reason to join the stormcloaks. Let's think this through. The Nords HATE elves, and beats races aren't even allowed in the cities aside from argonians, who are still greatly discriminated against. High elves are the worst of the elves, and the Thalmor has united with the Imperials. So surrendering to the Imperials is the same as surrendering to the Thalmor. That's
not going to happen. Out of all the races, the Nords should not and can not by any means or honor stand down.
So that leaves Bretons, Redguards and
maybe orcs (they
are mer) to join them. They're flipping off everyone else, which truly limits incentive to join them in the first place. Even then, Nords have plenty of reason to join the Imperials. The only time I'd ever join the Stormcloaks is if I'm playing as a Nord or
maybe and orc. The Redguard and Breton have much more reason to ally themselves with the empire for trade and political purposes. So my main character is an orc, and there's little pride in joining them. I don't feel like I'm a part of the fight. I'd rather go back to my tribe outside of society, or carve my honor with the Imperials.
So after truly going through it, ONLY a Nord should fight for the Stormcloaks, and even then there's a 25% chance that I'd join the Imperials and stay with the empire, but that would be highly dishonorably for a Nord to do so. Cowardly, even. If I'd join, it would be for the empire regardless of the Thalmor. But that's what makes a Nord, a Nord. He
MUST join the stormcloaks for the sake of his people.
In conclusion, I'm usually not joining any side, and if I am I'll
always join the empire unless I'm playing as a Nord, which is my least favorite race, so it's unprobable I ever will.