Chimney Bluffs is indeed beautiful and has some lovely camp grounds nearby. It is further away from Letchworth than you think though. It is as far north in NY state as you can get at that point, and Letchworth is about as far south as you can get at that point. There is also a considerable diagonal distance between them.
Having done both, I suggest that Letchworth is the more scenic and valuable of the two photographically, and the easier to reach from PA.