A Shkadov thruster is a megastructure which reflects all light from a star in a given direction.
What reflects more radiation, a white or a mirrored surface?
There is no such thing as a perfect mirror. Depending on the material, it will absorb and retransmit some frequencies, while reflecting others (with some absorption loss). If we are talking about all incoming frequencies, you'd need the know the breakdown of the spectrum in order to design a suitable mirror.
A white surface absorbs all (in a manner of speaking) light and retransmits it in the visible spectrum. If your incoming spectrum is mostly UV and above, that's a major loss in terms of energy. If incoming light is mostly visible, the loss may be insignificant. EDIT: This assumes that the surface is perfect white, i.e., retransmits all visible frequencies in equal proportions. Again, no such material is known.