If you don't get that NROTC scholarship, and you decide to still pursue the Marine Option NROTC, you can still compete for a 3-year scholarship and if you still don't get one, compete for a 2-year scholarship.
So just because you don't get a full ride scholarship doesn't mean you're footing the bill for the full 4-years
However if you not going into the Marine Corps under the MECEP with an Aviation Guarantee (a separate program but they end up going to college with the Marine Option NROTC mids) your first chance to put in a dream sheet for aviation is at TBS after you commission as a 2d Lt. and whether you get aviation or not is based on how well you do in TBS. Nothing from NROTC counts other than you got commissioned as a Marine Corps 2d Lt instead of a Navy ensign.
While at NROTC there is some flexibility between switching from Marine to Navy and vice versa. And switching to Navy you can try to service select Naval Aviation and you'll know if you're going into aviation or not before you commission as a Navy Ensign. You just won't know your airframe until later on.
So hopefully this helps in deciding between WOFT and NROTC.
Just because you don't get a NROTC scholarship doesn't mean that door to rotary wing aviation is closed.