If you aren't prepared now then I can't see you getting prepared in the next 2 weeks. So I would say take it in August. I mean, at this point you should be winding down the bulk of your studying and mainly working on practice tests. If you are still feeling like you have to madly study, then it is wise to wait.
However............ Ok, now I am sure people will disagree with my next point, but I am going to say it anyway. Have you done any practice tests? Have you studied for it at all? If you have then I would say take the April test. You may surprise yourself and score well enough to apply and then you would be done with it. If you don't do well, you will have another chance during the same application cycle.
Now, if you wait to take the August test and somehow do poorly since it is your first time, then you will be out of luck and will be applying with that score good or bad.
I know you aren't supposed to take the MCAT as a practice, but sometimes it is helpful because there is nothing that compares with the real thing.
Just my 2 cents.