Although there is little or no evidence in the film that Tessio was smarter, there is such evidence in the novel. Vito instructs his capos to not associate even socially. "He [Vito] explained this to the more intelligent Tessio, who caught his drift immediately, ..."

Of course, one could argue that in the film as well as in the novel that from all appearances the Corleones were doomed and that Tessio has assessed the situation correctly while Clemenza didn't have a clue.