Stormcloaks. The Empire is pretty much dead already. The Summerset Isles, Valenwood and Elsweyr are all under Thalmor control, the Black Marshes have seceded from the empire and conquered Morrowind, Hammerfell was abandoned to the Thalmor by the Empire and reconquered by the Redguards without their help, and Skyrim is wracked by civil war. The only provinces really left to the Empire are Cyrodiil and High Rock.
The Empire is simply too caught up in politics and bureaucracy to realize they've already lost. In fact, I'd guess that with as badly overstretched and universally despised as the Thalmor have made themselves, the Empire is the only organization left in Tamriel that couldn't defend themselves against the Thalmor.
The fact that Ulfric is ambitious and racist is hardly relevant at all. The empire needs to die before it can do any more harm than it already has. Also, comparisons between Ulfric and Hitler are just downright silly. Ulfric may not like the Dunmer much, but I can hardly see him trying to round up every single one of them in all of Tamriel to torture and murder them. All racism is bad, but holy crap people, there are many different degrees of bad, and Ulfric is nowhere near Hitler.