I think in order to answer this question fully, we have to make use of information we don't have:
1. What is the
real racial makeup of Skyrim? (I don't mean "count up all the NPCs in the game", either.)
2. What fraction of Nords either (1) are themselves racist, (2) don't care about racism, (3) think securing Talos worship is more important than racism, or (4) fear what Ulfric would do to them if they didn't side with him?
3. How racist
are the Stormcloaks,
really?
4. How willing is the Emperor to prosecute the war, considering he only sent one legion to Skyrim in the first place? Moreover, how willing would he be to sacrifice the integrity of the border with the Aldmeri Dominion to fight a Skyrim that, if the Stormcloaks won, would still be quite willing to fight the Aldmeri Dominion?
5. How many spies does each side have?
6. How defensible are the entrance points into Cyrodiil from Skyrim? A huge southern border means nothing if the bulk of it is impassable mountains.
7. How long will the Thalmor maintain their covert support of the Stormcloaks, especially if the Stormcloaks get the upper hand in the war? If the Stormcloaks get the upper hand, the Thalmor withdraw support, and then the Stormcloaks start losing again, will the Thalmor resume support?
8.
Who wins the battle of Whiterun? I've played it on the Imperial side (as well as the Stormcloak side), but have yet to kill off the invading Stormcloaks before they compromise the barricade and drawbridge - meaning they'd
technically already be in the city and high-tailing it for Balgruuf; it's just that in the game as soon as you pick your side you're assured victory.
9. How defensible are Markarth, Solitude, and Windhelm against starve-'em-out sieges?
10. How liable are the Silver-Bloods/Black-Briars to stage a coup in their respective cities?
On the surface youd say the Empire but realistically its internally dying on its ass and they look vulnerable to defection if the Stormcloaks racked up a few victories....so id have to say the Stormcloaks with an outside chance for the Empire as a last hurrah....
11. To what extent is the Empire really dying? The entire Fourth Era has been characterized by the theme of Imperial fragmentation - first the Aldmeri Dominion, then Morrowind/Black Marsh, then Hammerfell. If Skyrim does secede, High Rock would likely not be that far off.