I couldn't answer. The team is way more in shambles now but as QB's go Grbac was a major douche with no talent and Cassel just has even less talent.
Well after typing that out I have to say Grbac is my answer. Cassel was just a product of Pioli and those who were supposed to help him. He really had no choice but to go out and do is best, unfortunately is best was pretty bad.