Question about Opacity/General visibility

[ROUBAL]

Technically, is there a difference between setting the opacity of a material to (for example) 0.6, and setting the opacity to 1 while using for the object an Object Layer node with the General visibility of 0.6 ?

I also encounter a kind of shadowed area between two textured surfaces with alpha, while they both have an Object Layer with Shadow visibility disabled. It looks like some Ambient Occlusion... but I use pathtracing.

It is the dark line separating two surfaces at different distances (see the red Arrow). The actual color of this dark area should be the color of the gras which is just closer to the camera, shown by the yellow Arrow. It is a copy of the same image with different elements on it, but the grass is identical.

The scene is lit by the daylight system. The clouds are mapped on a cylinder with alpha surrounding the scene. The cylinder has an Object layer node with Shadow visibility disabled.

An idea about this shadow lane ?