Chelsea Manning was a confused person when she released the documents. She felt isolated from the people around her and searched for inclusion elsewhere. She developed a friendly relationship with Wikileaks personnel, and sent them hundreds of thousands of random documents. Later, she sent the helicopter video and hundreds of thousands more random documents.
That's not a principled stand against what the military is doing. That's a person unhappy with where they are lashing out and in the process occasionally hitting the mark.
The military subsequently mistreated her, and she should never have been in the position to do this in the first place. She ended up telling people that she was the one who leaked all the documents, when she didn't have to say anything at all. That doesn't make her a crusader or hero.
Snowden, on the other hand, was valued and paid well, and sacrificed that. He specifically targeted information that he believed was unconscionable and unconstitutional, and was careful about who he trusted with it and what he released. He had a well-articulated and thought-through purpose and has effected change through it.
