If you are prepared, take it in April. That has two advantages. If your application is in early and your MCAT scores are good and available early, and your recommendations are in early, you will be screened early. If offered an interview, you will get an early appointment. The class is empty early.
If your MCAT scores are not high enough you have an opportunity to retake in August and still be considered. If you take for the first time in August and don't do well, you can retake in April, but those scores arrive too late to be considered for that year's class. You will have to reapply for the following year.
However, I think it is foolish to take such an important exam when you feel unprepared. Confidence in your readiness to take plays a role in doing well. Then it makes sense to defer sitting for it until August. Even if you have to retake the the following April, so what? Even if you are in your 30s or 40s,
taking an exam you are unprepared for, though you might only think you are unprepared, might not get you an acceptance any earlier. So you may start medical school a year a year later, so what?
A rule of common sense applies here and in all aspects of preparing for such an important goal as becoming a professsional, whether physician, dentist, optometrist, podiatrist, veterinarian, pharmacist, nurse
etc.
Hysteria over starting medical school at an early age was always present to some extent in the past, but I believe it has been exacerbated by the sea change in the demographics of the college population in recent years. In almost all other countries medical education begins at an earlier age than it does in the US and Canada because in both countries the completion of a first, college degree is commonly required. Typically, four years to reach that level. Then, for the lucky ones, another four years to the MD. Then, three or more years of residency training. Many recent immigrant
families have difficulty adjusting their mind set to the new situation. Especially parents; everywhere, the older you are the more difficult it becomes to feel comfortable in new circumstances. The pressure to hurry places a burden on the younger generation; youth is more adaptable than age. I have seen some good students destroyed psychologically by the conflict in traditions. Of course, this pressure is not restricted to new immigrants, but it appears commoner there.
A very long response to what was a simple question.