The best way-and the most difficult-is a 4 yr. scholarship. Good luck, as there are very few-only a few hundred get accepted each year and those that aren't used do not get regenerated. The Marine Options are part of NROTC and are considered mid shipmen and some of their courses are the same, but they are different-Especially in the PT department. It is easier to switch from Navy to Marine Option if you are in NROTC already. However, it is doable if you are in another ROTC program-provided you have what they are looking for (GPA, PFT etc). As far as going the other way (to Navy) I was told the Marnies do a pretty good job at screening their candidates, so it rarely happens. As you know you cannot apply for Marine Option and Navy---it is one or the other.
Good luck and go for it!