Difference of the rendering

The top image was redered by MentalRay with Ray Trace(Final Gather) and Depth Map Shadow and took 1 minute.

The bottom one was rendered by Mental Ray Only with Depth Map Shadow and took 24 second.

It not easy to compare since the bottom one has more contrast and dark.

Final Gather calculate the reflection; therefor the scene tends to brighter.

But compare the whale in these pictures, the final gather makes more realistic relation between floor and whale.