The obvious cost is the need to render the scene geometry twice instead of once. An additional cost is that the deferred pass in deferred lighting must output diffuse and specular irradiance separately, whereas the deferred pass in deferred shading need only output a single combined radiance value.
The obvious cost is having to render the scene geometry twice rather than once. An additional cost is that the deferred pass in deferred lighting must separately output diffuse and specular irradiance, whereas the deferred pass in deferred shading only needs to produce a single combined radiance value.