Interesting question. The statement demonstrates both traits. Many would interpret Proctor's declaration as a sign of weakness because he is saying that he will falsely confess to save his own life when others such as Rebecca Nurse and Martha Corey have stood strong against the pressure of the court. But, Proctor's original reasoning for being willing to confess is to be with his wife and children and to obtain an opportunity to redeem himself in his wife's eyes. Moreover, John knows that the judges and court do not truly represent truth and morals; so in lying to them, he reasons that it does not matter because they have no weight with God. What does it hurt to give a lie "to dogs"? The strength in John's statement is that he is now considering his family instead of his own desires and might save other townspeople through his confession.
Ultimately, John's final decision--to tear up his confession and go to the gallows--illustrates the most strength because he has forgiven himself and provides support for his dear friends, Rebecca and Martha. Elizabeth recognizes this truth, and this is why she cannot heed Rev. Hale's plea to convince her husband to do otherwise. For John, the "strong" action to take is to keep his good name, and even though today would not view John as weak for falsely confessing, John would have judged himself weak.