The GMAT exam delivered at a Test Center and the GMAT exam delivered Online are designed to focus on critical skills needed for business school. The format of the exam is the same regardless of how it is administered. Whether the exam is taken at a testing center or online depends entirely on personal preference; either test format has no objective advantage.
Before scheduling your exam, you should check with the school(s) you are planning to send your score to, to see if they have a preference between the test center and online options.
For more information about the exam versions, please go here.