I think the evidence on the Tardigrade was a little less black and white. Burnham and the doctor thought the Tardigrade might be sentient, but didn't have proof. What they could prove was a degradation in its health due to use of the spore drive. In a case where its sentience was certain, there is no way that Saru would have made the same choice. But since it wasn't yet known, in the balance Saru was willing to take a risk and, as he said, was willing to suffer the repercussions of his decision should it come to that. And afterward, he realized that he had probably made the wrong choice, and had pushed the boundary in the case of extreme need. That is why he apologized to Burnham and why didn't need to listen to the computer's assessment of his performance - he realized that he messed up and knows not to do that in the future. His whole story isn't one of a character making the correct decision, but making a hard one and learning from the results. Not every captain is expected to be perfect, let alone first officers. This is part of the story of Saru developing the traits, the decision-making that will make him an effective captain. And even if aggression were an neccessary component of being a good captain, I don't think you can say that Saru doesn't have any: in his recent interactions with Burnham he clearly showed dominance in making his orders and carrying them out. Just because he is a prey species, doesn't mean he can never fight back, or learn to fight back.