I thought they were very close in proximity. I know that Nosy Radama is an island right of the north west (like almost ever other Nosy in Mad.). Generally from what I have seen, the Nosy Radamas have a little more red in them, but I don't think anyone, no matter how much experience, could tell the difference between the two. It is like what is the difference between Northern Ambilobes and Diego Suarez? It is very slight and sometimes they look like each other.
But basically they are close in proximity and almost identical in appearance.
-chris