If we're comparing bog-standard Jim Beam and Jack Daniels (as opposed to say, Gentleman Jack or Jim Beam's Devil's Cut), then I'm going to go with Jack Daniels, which I find to have superior flavor and smoothness. Even for a habitual whiskey drinker, Jim Beam strikes me as crude and rough.
Plus, Jack Daniels on the rocks was Frank Sinatra's favorite tipple -- the Chairman traveled with an emergency case of the stuff and reportedly told a doctor that he drank an entire bottle every day. (When the doctor expressed concern and asked how he felt in the mornings, Frank replied, "I'm never up in the morning, and I don't think you're the doctor for me.") He was even buried with a bottle of the Tennessee nectar. If it's good enough for Frank, it's good enough for me.