There was a thread a few weeks ago regarding Service Selection that you can read regarding percentages of those who get their top pick. Can't remember exactly but I think the number thrown around was approximately 85% got their first or second pick. If I remember right my class had around 185 Mids want USMC ground and we had 155 spots. The percentage of Mids who can select USMC is dictated by Congress. My class was just over 17% and I think they are at around 21-22% now. I think I read that around 210 Mids got USMC ground this year. I would not be surprised to see those numbers start to drop with the USMC Reduction in Force. But as you can see in the other forums on this site other commissioning sources are all facing reductions.
The decision can only be yours, if your goal is to be a USMC officer then pursuing every path possible is wise. The Academy does an amazing job of preparing Mids for commissioning as Marines. If that is your goal and you do well academically, in summer training, and do the things needed to become a Marine (great physical fitness, Semper Fi club, Leatherbeck, etc) then I would see no reason why you would not be selected. Now remember even if you do get a contract for PLC, you still have to successfully complete OCS. As numbers tighten for spots, OCS can see high attrition rates to weed the field. I think the biggest difference is the experience. PLC is normal college with some summers doing OCS and some interaction with an officer recruiter. The Academy you are surrounded by amazing SNCOs and Officers from the Navy and Marine Corps, are living in a military environment, have great summer training opportunities, training during the school year, mentorship opportunities, leadership training and opportunities. Also if you come to USNA and end up getting really hurt or decide you want to be a submarine officer there are other commissioning options also. You blow out a knee at OCS, so long and good luck getting back after a year or two, its not impossible but hard. That is the key difference between the two. Did you also apply for an NROTC MO scholorship? The choice can only be yours, but remember to not only look at percentages, but what do you want in your college experience?