Odetta, now she would've kept after him no matter what.
I'm curious why this would be. If it's because she is less sensitive to people's nature (in this case Roland) than Susannah, then yes I think that might be true. If it's because she is more moral than Susannah, I don't think that comes into it. Odetta is a nice lady but she lacks the strength and tenacity of Detta and Susannah. In that sense she would be more likely to turn aside than Susannah.
However, I don't think Susannah turned aside because she was weak. I think it's partly because of what people have said, she knew Roland's nature. Partly because she knew that she was destined to die if she followed him to the Tower. This doesn't make her a coward, or heartless. She had put herself in danger on many occasions, even responsible -with the writer's help- for removing the last enemy before Mordred, namely Dandelo. But there is a difference with giving one's life for a good cause, and sacrificing oneself needlessly.
She sensed it was the end of the road for her. Even her dreams told her too, so she left. In short, she left because she was meant to leave, she knew her place in the story was over.
True Mordred was still a threat (and don't think he wasn't despite being sick. He came very close to taking Roland's life when Patrick was lulled asleep. Thankfully there was another undetected lookout willing to sacrifice his own small life.) But I think Susannah somehow knew that it just wasn't her ka (yes we get back to that again) to remove that particular threat. Why? The dreams. The Tower. Her special perception we've been speaking of.