I'm an Elder Scrolls noob, but figuring out who and what the Empire was was remarkably easy. They're just doing what empires do, which is attempting to maintain hegemony over subject nations (while not in a phase of conquering others).
The Stormcloaks are a bit murkier. I've been trawling various cities and sided with both factions in different saves, but I'm still trying to work out two things. First, how broad-based is their rebellion? They're certainly grassroots, but is the majority of Skyrim's population behind them, or at least in a slight amount of support of them over the Empire? Second, what's the motive force behind the rebellion? Is it just the insult that the White-Gold Concordat levied against the nation by banning Talos, or is it the genuine suffering of their people under the reign of the Thalmor-dominated Empire?
This ultimately would probably decide which faction I join once and for all, especially the second question. If the former, I'd go with the Empire, as in that case, the Stormcloaks are just hotheaded fools dragging their own people into further suffering for no good reason. If the latter, I'd side with the Stormcloaks, as this means that the Empire has truly lost its mandate to rule Skyrim no matter how many people might want it to stay, and that Ulfric can lead the nation into a truly brighter future.