Atmospheric landings first. Space legs will/would be awesome but consider: there's either going to be a huge amount of work involved, years building vast amounts of assets to fill out those huge starports, stations, installations, bases, cities, etc. (including atmospheric areas for those Orbis rings and bio domes) OR they rush it and it's all going to be samey corridors or whatever.
You could say the same about atmospheric landings, but if you start with barren Mars-type worlds, with sparse flora and fauna at best, then the tech in-game really isn't that far off it already, it's just a matter of adding weather and pretty atmospheric effects (some of which we can already see from a distance), and more things to see and do on the surface, and you have a great starting point. And most importantly, something that gives us a feeling of more variety; bright, diverse and dynamic environments that are a stark contrast to the blackness of space. What a breath of fresh air that would be.
Station and ship interiors, on the other hand, will probably have to stick pretty closely to the design principles we see currently, which means for all vast amounts of work that would have to go into space legs, we're still exploring environments that are pretty similar in tone to what we already have. As immersive as it would be, I think the player value vs dev time ratio is worse. At least for me. Basic atmospheric landings could add at least the illusion of a lot more variety for seemingly less time expenditure and developer effort. It makes sense to do that first, while working on space legs.
The day I can sit of the surface of a newly discovered moon, looking up at the vast, shadowed crescent of a ringed gas giant in a pale blue, cloud-spotted sky, I will be a happy man.