I don't know that one is more or less beneficial than the other; both have their pros and cons, and in my opinion, a program should offer both types of experiences to truly allow for a solid foundation of training and knowledge in its students.
On-site placements such as counseling centers are generally great because they're easy (for you) to get to, generally have motivated populations (other than those forced by the school to attend), and will usually deal with less-severe forms of mental illness (key word = usually).
Off-site placements tend to offer larger varieties of patient populations, treatments, and work settings. They can be more stressful and frustrating, but they can also teach invaluable skills such as communicating to interdisciplinary team members, asserting the value of your services, and working with "difficult" clients who face multiple life obstacles (e.g., low income, lack of transportation, multiple health problems, non-existent social support).
