Pacino always had a lot more range than De Niro who was good for a while at playing both quiet and loud psychos basically.
You think of De Niro's best (or most famous) performances - Mean Streets, GF2, Taxi Driver, Raging Bull, King of Comedy, Cape Fear - they're all basically variations on two roles, the button-down psycho and the manic one. He also occasionally strayed into playing ruthlessly competent men - Once Upon A Time In America, Goodfellas, Casino, Heat, even his most famous comedy roles - Midnight Run, Meet the Parents - are kind of tweaked versions of that character. He can't do warm, he can't really do likeable and he never seems particularly comfortable with women onscreen, unlike Pacino.
I remember reading an unauthorised biography of De Niro years ago that quoted sources as saying De Niro made a conscious decision to "sell out" in the mid to late 90s. Apparently he was sick of the fact that, while he had high critical and professional respect, he hadn't made nearly as much money as some of his peers. He also had burgeoning business aspirations to fund with the Tribeca production company, the Tribeca Grill and his investment in Nobu. On top of that he got divorced in the late 80s, had 2 kids to support in maintenance and remarried and had more kids to look after. That explains films like Rocky and Bullwinkle, Showtime, Analyze That etc etc.
And you know what? Why not? No one gives shit about Gene Hackman (a far better actor than De Niro in my opinion) for cashing in with the likes of Superman IV - the Quest for Peace, BAT 21, or seemingly endless John Grisham adaptations in between performances like Unforgiven and Crimson Tide. But then maybe that's because Hackman was always brilliant, even in a bad film.