I'd kill them all, single handedly, if I could, and take Tamriel for myself and start a real government. Rule those rebelious fools with an iron hand till they accept me as their god... just kidding. Well, not about the first part. I would take the Empire for myself. Kill the leaders of both factions and recruit those who accept my superiority, kill the ones who go hostile.
I'll use my awesomely awesome smithing skills of godliness to craft steel swords that do 200 some damage and armor that hits the cap for all my warriors. We will be unstoppable.
Not only that, but I'll have freaking werewolf army, being the Harbinger of Companions and all the yada yada hoo hoo. And a league of Assassins, to kill the whats left of the Elder Council and any other powerful political leaders. Aaannddd, a College of Mages, you know, to place enchantments on all my cities. And an underground corporation of Thieves, if I ever run into money problems.
I'll join with Hammerfell, while they fight the Thalmor, I'll march on the Imperial City. Once I have control, I'll send ambassadors to each province to bring back troops if they are with me. Then fight the Thalmor and eradicate them. Then deal with the provinces who did not have my back. After that we'll that a peaceful empire with no internal problems

I'll be the greats Listening Harbinger Mage of the Thieves to ever live, oh, and I'll be an Emperor.
But, If I must choose. The Stormcloaks.