This debate shouldn't be won on their peak films - more in the deeper stuff, which is where De Niro takes the biscuit... for instance, De Niro, has shown tremendous chemistry with Grodin, Crystal and Stiller in comedies. Even if that chemistry with 2 of them faded with forced sequels. He had more willingness (before losing his mind around Bulwinkle time) to take on riskier projects to marginally decent success.
Though Pacino entered the 'retirement' years better - Brasco, Insider and Insomnia were all decent stuff to get from him while hitting my teens. But like Irishman aside, it's been horrific to the point that I haven't really looked up a De Niro/Pacino film since like 2004... I've not heard of most of them.