I voted other. Since some cities will be allied with the Empire and others will be allied to separatist rebels (no negative or positive connotations it's just what they are) I'd think the Imperial cities would be guarded by The Imperial Legion and the separatists would have their own police force.
Um, there's no way the Imperial Legion will have much of a foothold in Skyrim after 200 years of a decaying empire.
One of the plot elements is a Skyrim civil war between cities that wish to secede from the Empire and those that wish to remain a part of the Empire.