See it's a really difficult thing to choose if you aren't Nord or Imperial since one or the other may hold a grudge or bias against you (I'm usually a Nord)
I've never joined the Stormcloaks because Ulfric and his generals are just.... I don't know they irk me big time. That is the only reason I've never joined them.
Ulfric is some guy who thinks he should be king even though I believe he is far from a King especially one who would be a good one. I feel as if he'd be the person who'd kill or imprison his political enemies. He doesn't deserve to be king and I won't follow someone who can't lead. He's a boss not a leader. Then on top of that you have the Cavemen generals, who talk like they've eaten rocks their whole lives, and think like it too. Essentially: They're "King" would be no good king, and their military is full of Cavemen.
On the other hand
The Imperials are wusses, who back down to the Thalmor. It'd be best to Go Nazi Germany here and take complete control. Talk to the other powers nearby who can help, make Alliances, and build up. Unite the Empire under one true banner and march forth and take some Filthy Thalmor heads. The Empire will never recover if they keep allowing the Almderi Dominion to spank them. Signing the White-Concordat and following it to the letter will leave the Empire a tattered ruin of it's former glory. Best take it now or not take it at all.
On top of that, Tullius is a good General, smart, clever, and a good fighter he reminds me of Julius Cesaer(I wonder why) A strong man who can get what he wants and can smash anyone but once he settles, the daggers of political and philosophical beliefs will be his new enemy. He can't hold a country together, he's a conqueror and a soldier not a Liberator and Country builder.
Just my thoughts.