If you join the Stormcloaks, you will get a weakened Empire and a free-but-weakened Skyrim (no Imperial aid, no soldiers for patrols, no trade routes).
I don't know why this is always assumed. If Cyrodiil won't trade with Skyrim as a free nation- only one under their control- then to Oblivion with them.
And don't think the Thalmor will help you where the Imperials used to; Ulfric used to be a Thalmor pawn but after a while became a liability and a nuisance, and an uncontrollable one, at that.
No one wants or needs the Thalmor's "help." The Stormcloaks are fighting the Thalmor. As for Ulfric being their pawn, that is only what they claim and we don't have any other evidence to back it up. The empire are much bigger pawns for their aims.
In fact, the only town that has a real Thalmor presence that I know of is Markarth. As for racism, if you listen to in-game dialogue you can pick up on things about Ulfric being racist and elitist.
And Solitude. Basically, anywhere the empire controls.
The biggest disadvantage to imperial rule is that their leadership is weak and corrupt, not willing to finish the job against the Thalmor, and they are cooperating in removing humanity's biggest ace in the hole- the worship of, hence strengthening of, Talos.