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It makes no sense to join the Stormcloaks as any other race than roleplaying as a "true" Nord.
The Empire wins either way, even if your character is joining the Stormcloaks and wins the civil war. The Empire win of the civil war will be canon; your character will most likely after completing the story of the Dragonborn DLC "mysteriously" disappear while becoming the new champion of Hermaeus Mora. If the Empire won the future timeline will not change much, but if the Stormcloaks won, because of your disappearance, Ulfric will not have his best warrior anymore and lose anyway. You remember the Skyrim intro? The Imperial backup will capture Ufric again and this time no Deus ex machina will save him.
Also the Aldmeri Dominion is already controlling the Stormcloaks behind Ulfric's back.