Board Thread:Skyrim/@comment-10116285-20130528015330/@comment-7407991-20130528022649

Either one. Ulfric is their sleeper agent, but they also have the Empire in their pocket. If you want to be "lore friendly" I'd say the Empire, because the Thalmor seems to be trying to hide the fact that they benefit from the Stormcloaks just as much as the Empire.