These both look good. I prefer the first with the clouds in the canyon. I think the shaded sides of the canyon may be too dark. There are a few ways to fix this.
The easiest would be to adjust the ambient light of the atmosphere. This updates quicker in the preview window, so the tweaking is easier.
Adjusting the gamma and contrast of the render settings can also may a huge difference. To really see what is happening, small renders need to be done.
You can also adjust the exposure of the camera. This updates quickly in the preview window. It can easily whiten out the colors. If I think it will be needed, I try to do adjust the exposure before I start coloring the scene.
