There areen't any real reasons to join any side other than oppinions on the side's reasons for fighting and stuff.
The Imperials are really kind in truth. they are the "good guys" in my eyes. The only flaw is something that only affects Nords- they signed a treaty that banned the worship of Talos (an "untrue" god in the eyes of Elves) to save the Empire from obliteration.
The Stomcloaks- well, the Cause is true but Ulfric is not. He doesn't really care about liberating Skyrim, or Talos, or anything. He just wants to be High King. This is what I gathered from characters- I even got hints of it from people who REVERE the guy.
So yeah. It all comes down to: Do you want to be red or blue?
OH, and one more thing that doesn't really affect a mage character anyways: If you join the Imperials you can get Ulfric's Clothes, but if you join Stormcloaks you can get general Tullius' Light Armor.