Ok let me take a stab at this. I was a USNA Marine Commission, so I did not go the NROTC MO route. But you are asking for service differences not school differences. School wise the biggest differences I think you probably have figured out. Marine Corps you will go to OCS. It is no joke, but MOIs do a great job preparing MO guys. Follow their lead and you will do just fine. Some of my great friends commissioned via Norwich NROTC MO or via PLC at Norwich. It has been sometime since I went through all this, but in at least then, Norwich did a phenomenal job with NROTC MO guys. Out of all the SMC guys I worked with Norwich guys were by far my favorite. But, that is just my opinion and you must pick what is best for you.
So Marine Corps... Marines first and foremost love being Marines. We love to talk about being Marines and it honestly will carry with you the rest of your life. When you commission unlike in the Army you will not have your MOS or future duty station. You will first head to Quantico for The Basic School (TBS). The goal of TBS is to make you a basic rifle platoon commander. You also learn Lt things. Honestly TBS is not hard. Academically it wasn't challenging and physically no one single event was all that difficult. The great part is you will know alot of your peers in and outside your MOS while in the fleet. When I was there single guys/gals were required to live in the barracks. If you come from the SMC or SA side of the house this won't be a big deal. My two room mates were ROTC guys and they struggled to adapt for a month or two. They were not used to the nearly 24/7 military environment. But its only 6 months. Once TBS is done you will head to your MOS school. These vary from 3 months to a year. Most fall in the 3-6 month range and can be anywhere in the US. Infantry school you walk across the street in Quantico, Logistics is NC, AAV is San Diego, Artillery is Ft Sill. When you are at TBS you will get your MOS about 2/3 of the way through the school. You will get your duty station very shortly after that. Depending on when your school is, you could end up checking into your unit prior to heading to MOS school, all depends on school dates. This isn't to common, but it does happen. The USMC uses a quality spread for MOS selection. So each class is broken into thirds and then MOS selection is done by the top person in the top 1/3, then the top person in the middle third, then the top person in the bottom third, then repeat. I am sure there is tons of data if this is a good or bad way to do things, but it is what it is. In everything the USMC does, its all about MAGTF operations. If you don't mind floating on a boat for months at a time, then the USMC is a good fit. MEUs are generally 6 month floats. They can be alot of fun and a chance to travel to the world. Are there bad MOSs in the Marine Corps? All depends on what interests you. Take a look at this website as it lists officer MOSs and their schools:
http://www.usmcofficer.com/the-basic-school/marine-officer-mos-list/. The USMC considers these as combat arms: field artillery, tanks, AAVs, ground intel, and infantry. Tanks, AAVs, Ground Intel generally have very small numbers for each class, normally 1-5 range. Everything is about the needs of the USMC. Infantry, Artillery, Logistics and Ground Supply tend to normally have the most spots. Remember the USMC's goal is to be amphibious in nature and be able to project power from the sea. We have sort of put this a little on the back burner for the last decade, but if you read anything about the USMC planning, strategy, etc we are going back to these items and really focusing on training on these items. Once you arrive at your unit you will either be a platoon commander or section type leader (for instance Supply Officer for a battalion doesn't necessarily have a platoon, but they have a section and a shop to run). You will be prepared for this and will get your feet wet quickly. You will learn from other more senior Lts on what to do or not do. Your SNCO will keep you straight. Keep your ears open, listen and watch alot at first. Yes Marines will joke around about butter bars, but once you "earn your keep" trust me they will call you "their Lt", defend you and follow you anywhere. Marines have a special esprit de corps about them. The USMC birthday is a special day. We know our history and love that the title is earned. Sure the other Services have esprit de corps too, but Marines just take a it a step further in my opinion.
The Army is much larger which can be good and bad. Yes, that means more promotions, larger variety of assignments. If tanks, armor and artillery are big interests to you then Army is a better branch for those items. There is just alot more of it. The Army spends more money on it. There are more units, equipment and assignments to be had in these fields. In the Army you will go to BOLC after graduation for your Branch. I believe you will also know your duty station prior to graduation also. There are some correlation between branches and MOSs. The USMC sends Marines to the Army Artillery, Tank and MP schools, so those are pretty easy to figure out. Things like Quartermaster in the Army are similar to the USMC's supply and logistics fields (these MOSs merge at the rank of Major in the USMC anyways). I worked alot with the Army while deployed. Just like anywhere I ran into good and bad officers. I know in the Army Cadets can sign up for longer contracts to get active duty, branch assignment and base assignment type stuff. The USMC does not have this, but SPCs at TBS will work with you to know your interests. Oh yeah, flying. If you get an air contract on the USMC side there are jets, helo and cargo options. If you branch Army aviation chances are you will be mostly helos (there are others, but very very small numbers). You can apply for an Air Contract on the USMC side once you get going in NROTC and have completed the ASTB and qualified. This will mean you to head to Pensacola after TBS.
Ok, I can go on for hours. If you have more specific questions more than happy to answer.