OP, there are numerous threads on this exact topic within the last year or so. Might try searching the ROTC and USMC forums. I know there are several. Ultimately you should ideally be selecting the service in which you want to serve in vice school or unit, but those are realities and your success will be dictated greatly on an environment that works best for you. What school and unit feels best? Also, the size of each unit probably varies. NROTC (MO) can be pretty small at a lot of schools. Small or big battalions, neither is right or wrong, but what is best for you? From an ROTC perspective, what Major or Tier were you give in the scholarship? From my understanding, changing from that major is difficult down the road and could jeopardize your scholarship. Did you put engineering, but maybe that isn't what you really want to do? NROTC (MO), from my understanding (kinnem please correct me if I wrong) doesn't care about your major. Also, you are guaranteed Active Duty for the NROTC (MO) unlike AROTC. AROTC you could end up going reserves, having this option is appealing to some and not to others. When you graduate from AROTC you will have your branch and know where you are going. If you go Marines, you will go to TBS and not know your MOS and duty station until 1/2 way through TBS. I think its important to research the MOSs that interest you and see the career paths. Most are very similar and others are not. For instance tanks is much large in the Army. In the USMC, there are usually 1-2 tank spots for entire TBS company. Army has Chemical Corps, the USMC does not (well we have it, but its called something else and warrant officers serve in those roles, not regular line officers).
I served with a ton of soldiers while I was a Marine. There are differences, mostly in culture and mind set. Marines are very passionate about being Marines. If you ask a Marine what they do they will tell you a Marine first. Ask a Navy or Air Force pilot what they do and they will tell you Pilot first and service second. Both services obviously very field oriented, but with the Marines you could serve on ships also. Marines have MEUs and you can be at sea for months on end. MEUs are a huge part of the USMC, good chance you would serve on one as a Marine officer. I also believe the Army selects for Company Command (if this has changed someone please correct me). In the USMC we do not. If you are a Capt, you expected to be ready for Company Command when the time comes. Army, do you select for XO also? Just curious, wasn't sure on that one. The Army is much bigger than the Marine Corps, that can be good or bad, all on depends on the person. Each has career level schools, expects a lot of their officers and be prepared for the field!