First, that was a good catch, DC!
Second, I think Sonny acknowledges that he's not as good as the Don when, after Tom tells him he should make the deal, he says that the answer is "no" until the Don is feeling better and can take charge. In the novel, he tells Michael, "I know I'm not the man [Vito] was...but when it comes to real action, I can operate as good as anybody, short range." That's a pretty good, and pretty accurate, self-description. I also think (and this is a reach) that Sonny might have hesitated to sit in the chair because he realized that his gaffe at the meeting with Sollozzo, Vito et al precipitated the attempt on Vito.