According to the adcom it doesn't make a difference one way or another whether you do on or offcampus. The interviewers all have the same set of critera for evaluation.
However, on-campus interviews are generally shorter, 30-45 minutes, while off-campus tends to be a little longer. This is purely a personal preference; in an off-campus you have more time to establish rapport but also more time to screw yourself up and say something stupid.
The only reason I chose on-campus was because I had not done a campus visit, so I coincided the interview with a chance to do a class visit to give me a little more ammo when they ask the "Why Booth" question.