The difference is in the highlights, in the C3D7 Cheetah render, the highlights or white parts retains detail and has more colour saturation.
In C3D6 render, the highlights or white parts are "over exposed", no details just white wash. Colour is also less saturated.
Compare the waterfall curtains and the overall brightness, both lighting and camera settings are the same in both examples.
It seems C3D7 handles light differently, it looks more pleasing, C3D6 seems more contrasty and stark in the examples.