How is Bassanio's logic in choosing the casket different from Morocco's and Arragon's?

You are essentially correct. Bassanio laments about how people are too concerned with outward appearances rather than the substance inside. He also thinks that, if this is indeed a test, nobody will suspect lead of carrying such treasure. Bassanio refuses gold because he knows that "all that glisters is not gold." He also refuses silver, calling it "common drudge 'tween man and man," as coins are. He chooses lead because he knows that true worth lies inside, even if the outside doesn't look like much.