Although you can achieve a clean result using final gathering by itself, there may be times when global illumination enhances the photorealistic effect you desire and eliminates shadow artifacts. Keep in mind, however, that it takes longer to render if both methods are used simultaneously.
Conversely, a global illumination effect is enhanced by final gathering. By tweaking the final gathering parameters, you can control the color bleed using fewer photons, hence making it faster. Final gathering also adds realism to the global illumination effect.
You can select the Fast Lookup - Irradiance from Photon Map option when you are using global illumination and final gathering together. This stores irradiance information for each photon location in the global illumination calculation. This means that you need fewer final gathering samples, and the final gathering calculations take less time. However, global illumination calculations will take longer.
