I downloaded Unigine Valley today and played around a bit (
https://unigine.com/products/valley/download/).
Versus Outerra, Valley has a certain initial wow factor that I think boils down to these factors:
* Valley's focus on impact visuals at the expense of realism (unrealistically large sun, moon, overdone color saturation)
* camera aperture and focus distance.
* lens flares
* rain, wind, moving fog, clouds
* detailed trees with moving leaves
* random rocks added to base terrain
* more varied greenery including different grasses, flowers, bushes
* floating debris -- dust, flower petals
* sun and moon cast shadows, rays over scenery
* more refined HDR/blooming implementation
Having said that, a few areas that I think Outerra outshines Valley:
* better management of detail at a distance, better looking non-detailed trees
* Valley's rocks fade in pixel by pixel in an obvious way as you near them
* Outerra's complete detailed world from ground up to space
* Valley's overdone and unrealistic leaf/grass movement (even in zero wind)
* Valley's repetitive distant mountain textures apparent when aperture tightened and focus at infinity
* Outerra's biomes, extensibility, flightsim/gaming components, etc.
* Valley's unrealistically large stars at night