Both of these images have GI, which is why they look very similar. Did you mean that you enabled "GI surface details"? That is just an enhancement which calculates additional GI detail or Ambient Occlusion details in the final render pass, mostly to improve contact shadows and improve crispness of the GI or Ambient Occlusion at the smallest scales in image space. It adds a lot to render times, so I would only use it if cropped tests show that it really helps an image. To disable GI completely, disable the Enviro Light.
Matt
