I've been on the Apollo 15 Expedition these last few weeks.
I've been travelling and exploring in Odyssey and going to the expedition meetups in Horizons.
What I've observed is that when you're actually on the ground (or very close to it), in most cases, Odyssey looks much better than Horizons. Much more variation in local terrain types, terrain that looks like it was forged by natural processes. I've found things that look like U-shaped valleys (results of glaciation), things that look like geological layering etc. I've been to places that are highly evocative, such as one that looked like I was on the salt flats in Utah, and another which due to the lighting and the materials of the planet, was almost entirely monochrome with the only splash of colour being my landed ship. All these looked great. Overall, I found the "on the ground" experience (modulo the bad frame rates) better than in Horizons.
However, what's really jarring is how much better Horizons looks once you climb more than about 1km above the surface in your ship. The topography whether you're looking at it when entering orbital cruise, or at medium altitude (e.g. 3000m) looks so much more convincing in Horizons in most instances. Certainly there are places in Odyssey where it looks nicer when you're at 3000m, but more often than not, the Horizons topography is so much better. In Odyssey, from orbital cruise, you often have a strange sense of deja vu, because the large scale feature you're seeing ahead is exactly the same as the large size feature you saw ahead in another star system, because it was hand made and there are only a few Slartibartfasts but billions of planets. You don't get this sense of deja vu in Horizons. It's not just the large features though, from a few km up, Odyssey often looks a bit like those 1980s height map tech demos people had done on the BBC Micro (only, of course it's now filled in and textured). It looks a lot more artifical and "height mappy" than a lot of Horizons topography.
If only they could have Horizons style topography with Odyssey style views on the ground, it would be excellent.