I like them both, I prefer DeNiro when he was younger his performances in Taxi Driver and Raging Bull for example are incredible. But I think he tends to play Robert DeNero these days. Heat was his last great performance but that was over 10 years ago......Rocky and Bullwinkle, Hide and Seek err we won't go there.
Pachino, possibly has the edge for me. One of my favourite films ever is Serpico and he is just incredible in it. He is amazing in Scent of a Woman playing a blind man and would Scarface have been anything without him.
I dunno you have to go back and look at their older films to see them at their best. Both legends non the less.