GMAT and GRE are completely different tests. So once you decide which exam to take then don't look back. Here are some things to consider:
1) School numbers: Most business schools accept GMAT and some accept GRE. You shouldn't limit your school choices based on the exam so my recommendation is so choose GMAT to keep your options open.
2) Post MBA: Gmat prepares you better for management post MBA as it deals with time management, attention to detail, organisation, analytical and problem solving skills needed to be successful in a business environment.
3) Aptitude Tests: Gmat is more aligned with aptitude tests than GRE. So it will come in handy when you are applying for highly competitive jobs after MBA.
4) MSc : If you want to keep your options open for both business schools and science Masters, go for GRE. That's because GRE is the required test for most MSc programs.