If what you want is thrust, then a solar sail has got to be more efficient than a solar cell driving some engine.

Simple proof:
If you use a solar sail, you can reflect the photon off of it back in the direction it came. This means you gained the momentum of the photon as you stopped it, then you gained as much again as you threw it back behind you.

If you use a solar cell, the best you can do is match the first step in catching the photon, and absorbing its momentum. After that, there is no lossless system which transports / changes the form of that energy.

No matter what happens in the process of turning the captured energy of the photon into thrust, it cannot be as efficient as simply bouncing the photon back in the direction it came.

This model assumes you are oriented such that the desired direction of thrust is exactly away from your light source, but the conclusion is true for any angle.