Once you choose a side, that's it, no switching. The rewards are mostly the same, except for whichever leader you kill, you get to loot.The Stormcloaks are a bunch of racist Nords who want to make Skyrim for Nords and only Nords. If they win, they'll probably eject non-Nordic humans from the province, and murder everyone else(after the game, nothing happens during). Also, Ulfric is either an active Thalmor agent, or is 'merely' compromised in someway.The Imperials are the sad remnant of a once powerful, centuries old Empire.
Curious, I made the 'Join the Legion' quest active and the map marker arrows directed me to the Fort prison, wherein I found three arrows just floating in thin air over nothingness, each indicating it was part of the 'Clear Out Fort Hraggstad' part of the quest. For The Elder Scrolls V: Skyrim on the PC, a GameFAQs message board topic titled 'Can't join Imperial Legion'. Skyrim: How to join the Imperial Legion Army Faction. The Imperial Legion is a part of the armed forces of the world of Skyrim that’s led by General Tullius. Some in Skyrim might have secession on their minds, but the Imperial Legion are quite against that. If you are too, you can join them.
They are trying against all odds to hold their vision of Tamriel together, but after losing a war to the Thalmor (who are essentially fantasy Nazi elves), the provinces are crumbling. Skyrim is one of the most stable, and therefore most profitable still remaining.
Their interests lie in Cyrodill first, and they will gladly exploit Skyrim for everything it's worth to keep the Empire lurching on for a few more decades.