I haven't joined either one yet, they both seem like dicks.
The Stormcloaks are racist, shortsighted thugs with a superiority complex and all the organization of a frat house.
The Empire are spineless, unscrupulous, bureaucratic puppets, whose response to every problem is to disenfranchise their citizens and suck up. They're willing to bend knee to Nazis to avoid having to stand up for themselves.
Quite frankly, the Dark Brotherhood probably do more good then either of these organizations, at least they act reasonably faced with conflict. That, and assassinating you know who could end up doing more good then any amount of civil war.
A nation and king I could get behind?, The Forsworn. It'd make an awesome quest, too. Who doesn't want to roleplay Braveheart?.