I believe it comes down to athleticism, shooting ability, and potential. James Young has all around better athleticism, is actually shooting better now, and is showing some growth.
RJ Hunter came back in year 2 and he was still getting outmuscled and his shot still wasn't falling at a markedly better rate. I feel he probably can still make it in the NBA as a bench shooter, but he just wasn't good enough based on the roster crunch.
Best of luck to Hunter in the rest of his career. If he makes good, a return to Boston later down the road isn't out of the question as seen by our own Gerald Green.