Summary: The only reason why both of these movies are being bundled together is the presence of the Irish mob in them. However, as far as gangster movies go, they are both idiosyncratic on their own rights.
Unlike The Godfather or Goodfellas where the benchmark for gangsterism in films were set, both of these movies concentrates on the "grunt" level of the mob. It's ground zero. Both movies feature a protagnist caught between conflicting conscience and interests.
Miller's Crossing shows a distinctive Coen style of story telling. Gabriel Byrne shines here, and should have gotten an Oscar nomination. Road to Predition, a prohibition-era take on the Japanese series, Lone Wolf and Cub, takes American everyman, Tom Hanks, into the role of a mob hitman. The crux of the film is on the relationship between the stoic "old-schooled" father and his son. Something of noteworthy: Conrad L. Hall's swansong of a cinematography won an Oscar. A picture perfect close to an extraordinary career.