If I were to join Army ROTC and participate in SMP, would that lessen my chances for a USNA Appointment? Or would that be considered the same as NROTC participation?
My understanding is that if you are in the Army or AROTC you cannot use your service nomination for a different service - i.e. Navy
If you go to school and take NROTC you are eligible for a ROTC nomination to Navy.
This is the only disadvantage I can see.
Be certain you understand the ramifications if you do not get appointed.