The question is which one do you need to sit and that depends on what you want to study. The GRE is the preferred admissions test for most non-MBA graduate programmes, however, GRE is accepted for some MBA programmes. Certain MBA programmesonly accept GMAT. Therefore, if you are unsure of what you want to study, or plan dual-programmes, it may be better to write the GRE exam and cover both bases. However, if you know you want to do an MBA it is recommended you take the GMAT, as business schools do consider GMAT scores show commitment to an MBA. In fact most MBAs require GMAT regardless of their ranking. In addition, a good GMAT score will help your future career in the business and finance sectors, as potential employers will take GMAT scores into consideration.
In summary, if you know where you are applying then check with the school. If you are unsure of the school then look at your intended major. If you plan to do an MBA then write GMAT. Still unsure, then you may decide to write both.