Torvald and Krogstad are both dislikable characters who have a façade of strength and dominance about them. For both of them, strength is tantamount to bullying. Krogstad bullies in a sniveling way through attempted blackmail. Torvald, however, is a conventional representative of the system, of society. He seems obsessed with his own righteousness and the supposed fact that he's an exponent of morality and order. It's unjustified power, not "strength," that he exhibits. In a contest between Torvald and Krogstad, it's somewhat immaterial which is the stronger character. But if forced to choose, I would almost have to say the unsavory and slimy Krogstad is ironically the better, or rather the less negative, exponent of strength.
Krogstad is at least consistent in his campaign to secure some kind of breakthrough in his own life, to reestablish the successful career he thinks he deserves—despite the nearly criminal way he goes about it. He acts with ruthlessness, but he's not exactly a hypocrite. We learn that his business life and his reputation have already been destroyed because of the "indiscretion" he committed, which is the same thing Nora did, though up to the last scene she's gotten away with it. Torvald, on the other hand, has evidently never done anything society judges wrong. Even his criticism and dominance of Nora are what would have been considered normal in the nineteenth century, and later.
Yet this doesn't indicate any real type of strength of character, or of anything else. Torvald shows himself a complete coward when he learns of Krogstad's threat to Nora and, by extension, to himself. His vicious outburst at Nora, though an exaggerated version of the way he acts toward her in general, is rooted in his own fear for himself. He's so aghast at the prospect of his own career and reputation being destroyed that he launches into a stream of abuse at his wife (his "little skylark"). When the threat is defused, he prefers to congratulate himself on his supposed tolerance of Nora's imperfections rather than to apologize for his behavior. At least in my view, that makes him even weaker than the corrupt Krogstad.