Hi johnsonn,
Many Business Schools now accept either the GMAT or GRE score as part of the application process. While there's certainly an advantage to taking whichever Test you're more comfortable with (and increase your chances at a high score), you should do some research to confirm what score YOUR first-choice School expects you to provide. As an example, if that School only accepts the GMAT (re: it doesn't accept the GRE), and the GRE was the one score that you had, then you would face an odd dilemma - either don't apply to that School or spend more time studying and take the GMAT.
This is all meant to say that the test-taking and application processes CAN be planned out in advance, but you have to clearly define YOUR goals and timeline first (if possible).
Do you know which Schools you're interested in?
Do you know when you want to apply?
GMAT assassins aren't born, they're made,
Rich