I'm not saying that I know the answer to this, but here's what I think.
1. The MCAT is graded on a curve. How well you do depends on how well everyone else does.
2. A vast majority of people who take the MCAT have more time to study in June/July than they do in February/March.
3. Regardless of which test you take, you are scored relative to other students who are taking the test on the same day. You might have studied more for the August test, but so did everyone else, so I figure it evens out.
4. I bet the April MCAT is easier than the August MCAT. At the same time I think that getting a higher score on the April MCAT is NOT easier than on the August test.
Taking the MCAT is so much fun that I suggest trying both and deciding for yourself!