I hate them equally. The Stormcloaks are comprised of xenophobic Nords who want to purge the land of any people aside from themselves. This is offensive on many levels, not the least of which is that I rolled a Breton.
The Imperial Army on the other hand, is ruled over by a cowardly emperor who lacks the self-sacrifice one would need in such a position. The Empire also forces harsh taxes onto the people of Skyrim, which apparently a lot of people can't pay.
Although I'm not sure about the last part, after all I've been in dozens of houses and seen the valuables that people keep. Those valuables are mine now.
I think I'll side with the Imperial Army when I get around to it, the Stormcloaks methods would cause years if not decades of problems, whereas all we need to do is wait for the Emperor to die and hope we get a better one.