A lot of factors go into making that decision-
Technical Requirements
Environment Requirements
Writing Materials
Exam pattern
The pros of GMAT online are-
You can take it from home and control your environment
You can send your scores to universities for free
The test doesn't have an AWA section
It's cheaper
The pros of regular GMAT are-
The environment and set up is ready for you at the test center
All business schools are accepting these scores
You'll get a break between quants and verbal sections
You can re-take the test
You can read the details here.