Zidane is a God. It's not only what he accomplished @ club level, but also on an International stage. Guiding France to the World Cup in 98, winning Euro 2000, and almost single-handedly leading the French to another triumph in 2006. Del Piero was always a peripheral figure for the national team; in 98, which was supposed to be his tournament, he was outshone by Baggio in the colors of the Azzurri. He never really had the same impact for Italy that Zidane had for France, even in winning the World Cup with Italy in 2006.
Even at club level, where both players have an abundance of accolades, Zidane won Real Madrid the CL trophy in 02 with that magnificent strike, and played at the highest levels more consistently!
Had Del Piero not suffered that terrible injury, things might have turned out differently, but as it stands, Zidane is clearly the better player. Del Piero never truly recovered from that injury.