To let you know my perspective, I am an OCC 187 Applicant and plan on being in Quantico in Oct. I have just a little over 100 hrs flight time (PPL - Non-instrument rated) mostly in Cessna 152s and 172s and a little time in a 1960 Mooney. I have no military experience and I don't pretend to know much about military flying for the simple reason that I don't know jack (I've read all the books, though).
My advice to you, and take it with a grain of salt, is to get an introductory flight from your nearest general aviation airfield. Cessna offers a $49 intro flight at their pilot centers (
http://learntofly.com/howto/discovery.chtml). If there isn't one near you, check the yellow pages for flight schools/training. A lot of mom and pop places may take you up for free. I earned my certificate from a mom and pop place - best choice ever.
Once you take your flight, if you are smiling from ear to ear and people look at you strangely because you have a goofy look on your face, go air. If you had less than a perfect time, go ground. If you have already flown before and you asked the question, go ground.
I've read it on this forum before, and I agree, you want to be a Marine Officer first, pilot second. From my own limited experience in flying and in life, you will never make it as a pilot unless flying flows in your veins. You have to really want it.