This depends on whether or not you have a vacuum in the radiometer:
The photons hitting the black side of the vanes will be absorbed transferring their momentum to the vane. Those hitting the white surface will be reflected transferring up to TWICE their momentum to the vanes.
1) In a vacuum: The above concept dominates and the white vanes trail the black vanes.
2) In a poor vacuum: the air on the black side of the vane gets heated and the air molecules give an extra "kick" to the black vane side overriding the photon momentum transfer causing the black vanes to trail. I.e. the air molecules transfer more momentum to the vane than the photons do.
Pete Karpius, Physics Grad Student, UNH, Durham
'Never regard study as a duty, but as the enviable opportunity to learn to know the liberating influence of beauty in the realm of the spirit for your own personal joy and to the profit of the community to which your later work belongs.'