Well some agency's have "stadard caliber(s)". For example, if i wanted to be a FBI special agent i could pic between a 9mm, .40, .45 ext. As long as i qualify on the range with my service weapon in one of those calibers. Its realy a matter of personal preference. Although some agency's might have only one choice.
P.S. 9mm, .40, .45 are the only ones a Government agency gets free for training and range time.