Well... *sigh*
I've had some more time to think on this and rather than come up with some humungous proof, I think it boils down to this.
Stormcloaks were created to serve Ulfric and his interests. Now if you have no problem with this and are firmly behind these interests, then Stormcloaks are for you. However, you ultimately answer to Ulfric and are sworn to serve him, "Even in death". It's almost as if Ulfric is telling people to not worry about the Gods judging them, that he is the only judge they will ever have to answer to.
Imperials are playing by the numbers and are trying to walk on water, make off with the warden's daughter. Imperial interests are geared towards the good of the Empire first and foremost. Which means, these interests can change and as an Imperial... you have to be able to adapt and accept change. However, what I find so compelling about the Imperial oath, is that you are not swearing yourself to one person really. You are saying that your conscience is clear and your actions are freely able to be judged by both those above and below.
Of course, both of these entities are now servants of the Aldmeri Dominion, which serves the Greater Good in Tamriel. Furthermore, the word of man is empty and void, much like the oaths they take to their archaic Kingdoms and decadent, divided cultures.
ALL HAIL THE ALDMERI DOMINION
ALL HAIL THE MADGOD