Not just who, but when Not just who, but when In considering someone's research activity, you may also consider where they are in their career. People early in their careers, assistant professors, are just beginning to establish their research program, but they are madly trying to publish as much as possible, so they can get tenure, and become associate professors. Associate professors typically have tenure, and have established themselves and their research programs.
Full professors have tenure, established research programs, and often considerable reputations.
However, their reputations may be based on research that was done some time ago, so they may not be as active currently. The former would suggest that that's what you will be doing when you're there, while the latter suggests the avenues of research open to you there would be greater. Junior profs may not be as well connected and their letters of reference may not carry as much weight, but they may have more time available to devote to grad student training.