About Jake choosing Cassie over the world... I think it's still ambiguous as to whether or not he made that decision. Actually, between choosing the person you love and the world, wouldn't most humans pick the former? What sets Jake apart as the leader is his ability to make the tough calls for the greater good, and he does sacrifice Rachel like that in the end. The comment about his choice being interesting could be in reference to that rare way of thinking/drive to actually do it. The reason why he calls Cassie afterward could be because he's aware of this side of himself and, similar to how Rachel wants Tobias to be her link to humanity, Jake wants Cassie to be his. I love the similarities between Jake and Rachel, subtle as they are.
Anyway, I'm not saying I disagree that he chose Cassie, I'm just saying it's not all that clear.
I do agree that it wasn't Cassie's responsibility to save him in the end. I did find it a little bit odd, and rather sad, when she didn't help him until later when she had her own life sorted out--but that's fair, I think. You have to help yourself before you help someone else. She didn't leave him because he changed--she just wasn't ready herself, maybe. And she couldn't save him completely, no matter what. The fact that she accepted that in the end showed how she matured and healed herself after the war.