I do not know how the chances of getting Marine at USNA compare to the NROTC-MO, but I'd like to suggest that you look at this decision from the perspective of what would leave you with the least amount of regret in the end. In other words, what would make you the happiest? You can go to USNA, perform, and become a Marine or choose the NROTC-MO route, perform, and become a Marine. That's the best-case scenario for either route. You ought to decide which of these scenarios is the best for you. Which best-case scenario is the one that you want to happen? Which will leave you feeling the least amount of regret? Based on your post, it's clear that the USNA route is the one you would like the most and will likely leave you feeling the least regretful.
However, the USNA route has the risk of you not getting assigned Marine come Service Assignment (though, this same risk exists for NROTC-MO but I presume on the lesser side). But on the other hand, if you go NROTC-MO, there exists the risk of you feeling regretful for not going USNA and always wondering the "what if..."
Regardless of the route you choose, as those above me have said, you will have to perform to become a Marine and as long as you perform well, your chances are good. I presume that you have that drive to perform and do what it takes to become a Marine no matter where you go, so then it comes down to choosing the best-case scenario that you want. What is the best-case scenario for you? What will leave you feeling the least regretful?
Whatever path you decide to walk, I wish you luck.