Steel fibers have greater tensile strength than PVA fibers, but this strength is rarely used. Steel fibers, like rebar, are passive reinforcement, meaning they have no reinforcing effect until the concrete cracks. The problem then is that steel fibers tend to pull out rather than hold the concrete together. Many techniques have been used to increase the mechanical bond of steel fibers--hooks and waffles are among the most popular--but even so the steel fibers pull out easily. Contrast this with PVA fibers which create a molecular bond with the cement during hydration. When you try to pull a PVA fiber out, it holds on firmly, taking full advantage of PVA's tensile strength.