The reflections are 100%, since they are total-internal reflections. Any wavelength dependence would be in the anti-reflection coating on the front (left side) face.
Compare that to a mirror, where the wavelength dependence arises from the mirror's reflective coating.
So, the question of which is better depends on whether it's easier to make an anti-reflection or a high-reflection coating uniform AND close to 100% over a wide range of wavelengths. Anybody else have any thoughts on this?
I am thinking that since binoculars use Porro prisms, they are either better than a mirror or much cheaper than a mirror of the same performance.