They're both great actors and legends. But I agree with David 470, Richard Craig, and Tragger. Pacino seems to have slightly more range. They both play both good guys and bad guys; but Pacino does "good" guy a little better than De Niro; and I can't imagine De Niro could have pulled off Michael Corleone or Scarface they way Al Pacino did.
I'd be interested to see how more people answer this question myself!