Which minimizes solar heat absorption better, polished aluminium or white paint on it?
It would depend on the specific materials, but in general a good polished aluminum will reflect more light than a typical white paint. The aluminum should be well above 90% reflectance while the paint will be between 80% and 90% reflectance. You also have some control over where the reflected energy goes with the mirrored surface whereas it is reflected diffusely in all directions from a white surface.