It seems like the emission shader lights up other objects stronger in the preview render than in final renders.
I'm using mesh lights in Blender 2.78. The error is reproducible by creating a plane with an emission shader and a subject with any kind of material, that's lit by the plane.
As I didn't find any reports of this issue, I'm wondering if I'm the only one experiencing it, and if so, what settings might cause this.
It is a bit closer to the preview, but then I noticed that both of the lights are reflected two times. (Which was visible before, but I didn't realize it.)
I also tried to render with CPU, and the result was identical to the preview.
So it's definitely a problem with the GPU (Nvidia). It's unlikely a driver problem since I noticed the problem some weeks ago.