Don't feel like playing for the Empire again but I'm reading that the Stormcloaks are basically Skyrim's version of Nazi's and that an Orc would never join.
That is Imperial probaganda. Go to Windhelm, meet the man and make your own judgement. While you are there, you should take the time to listen to the conversations he has with his steward and with his general.
As others have said, don't believe in everything you read. Whenever someone (also in game) tell you something, it is their point of view. And they may have an ulterior motive (like getting you to fight their war), and their judgement may be clouded by lack of knowledge or ideology.
Thalmor want to exterminate Man.
It is my understanding that the Thalmor want to rule men, not exterminate them.
It is true that many Stormcloaks are racist, but then the Empire has plenty of sketchy supporters as well.
The vast majority of the stormcloaks I met were no more racist than any body of soldiers at war. And only few of their supporters were openly racist.
Stormcloaks are more like the American Colonies in rebellion from the Brittish Empire. Yes, there is a problem with racism in Skyrim, as there was in the colonies, but it's not the reason they're rebelling. The issue simply takes a back seat to the bigger problem, namely an Empire that does not have the best interests of it's constituent nations at heart. So, think of Ulfric Stormcloak as George Washington; Washington was a slave owner (though by most accounts, a good one) but that doesn't mean he wasn't a generally awesome guy and a brilliant leader.
Interesting comparison.
From what I gained from the Stormcloak:CW quest is that Ulfric Stormcloak himself is no racist. He's just an inspired Nord with a love for his nation who wants nothing more than for them to have a stable, self-determined state like they had in the old Empire. Ulfric doesn't realize it yet, but the Thalmor are his real enemy.
Ulfric fully realises it, but he also knows that in order to win, he needs not only a strong army, but an army that believe in what they are fighting for. In the eyes of most nords, the Empire lost a lot of credibility when they bowed down before the Thalmor and outlawed Talos worship. This means that nords may fight for the Empire against the Thalmor, but they will not really believe in the cause they are fighting for (the Empire). When fighting against the Thalmor for a free Skyrim, they have a cause that they can believe in.