Which surface reflects more light into the black collecting surface, considering both the light collecting surfaces have the same length? Is there an equation that could help me determine this? I'm not an optics expert, I work in radiation detection (I'm working on a scintillator, and I'm trying to optimize geometry, but I'm struggling with some stuff).
Using the cosine law aka as Lambertian reflection , yes. https://en.m.wikipedia.org/wiki/Lambertian_reflectance A Lambertian reflector looks the same intensity from any direction.
If light is coming from below, and assuming both surfaces are 100% reflective, light will always eventually escape through light collecting surface, regardless of its geometry. Even if the reflector is just flat.
I think you might want to clarify where the light is coming from and how exactly you quantify amount of light going through a surface.