Until we see planets with atmo in Elite, it's hard to compare the 2.
Nah, it's pretty reasonalby possible to compare them on a couple of topics, most notable of all: Geometry!
Outerra bends Elite Dangerous over and spanks it
mercilessly in terms of geometry. Overhangs, actual, sharp ridges on maintains, the whole lot Elite wishes it had in order for it's vertical features to look like actually convicing cliffs instead of the molten ice cream mountains we get instead, which look plain aritificial, imo pretty boring and unconvincing. Boulders of varying sizes also integrate so much better int Outerra's landscape than the somewhat unfitting meshes in Elite, that are just sunken into the ground..
The only thing I could come up with in Elite's defense here, is that it's a networked game. In order to prevent weird side effects from happening with players moving through geometry others can't see or floating through the air for others, everybody has to have the same geometry. It can probably reasonably assumed that adding Outerra's horizontal displacement in Elite would make the weaker PCs weep, on which Elite still runs more or less reasonably. But I'm still holding out for them adding the feature some time down the road, when the base level for gaming PCs has been raised.
In terms of production values however, it's very much visible that Outerra has no budget behind it. It's more of an engine than a game - smilar to space engine. The trees for example are flat 2D textures that just happen to always rotate so as you can see the flat side. Looks funny from straight above, but surpisingly convincing from some distance.