It highly depends on your MCAT score. If you get 33+ it won't really matter. If you get less than this, then you will be stuck in the middle of the application pack (competitively wise), but your application gets read at the bottom of the pack (late Sept. completion date). That latter is disastrous.
In your case, if you take it in August, I would just recommend not submitting until you know the score. Otherwise, you could be wasting a lot of money for a late (or no) interview that most likely won't turn into an acceptance at that point.
Golden rule:
If (MCAT) high ---> you apply
If (MCAT) borderline -----> you must decline (applying this year and just wait it out for the following in June)