The Complete Beginner's Completely Incomplete Guide to Oxygen Not Included
The base game compared with the Spaced Out DLC
How do they differ? Which one should you choose?
The short answer:
In Spaced Out, research is a bit trickier, power is a lot easier *on asteroids without comets* (once you get hold of glass), and you can travel to other asteroids.
The long answer:
As a general point: you can activate and deactivate the DLC whenever you want.
In Spaced Out you can choose between "Classic" and "Spaced Out" when starting the game. If you choose classic you start on a large asteroid, like in the base game (but with DLC stuff unlocked). This alternative lessens the need for space travel.
And now on to some differences between the base game and the DLC.
Space and other planets
In the base game you can send rockets off to other planets to gather materials but you can't actually visit those planets yourself.
With Spaced Out active you can visit a bunch of different planets. Your dupes can travel back and forth between them, build on them, ship resources between them, etc. Your starting asteroid is truly just your starting asteroid, and other asteroids (and new kinds of geysers) await.
However, (particularly if you choose the "Classic" start) space is also largely avoidable if you aren't hugely interested in that aspect of the game.
In the base game you can research everything except some rocketry stuff using just water and dirt. (Research in Oxygen Not Included is done to unlock new buildings.) To unlock the whole research tree you will have to send up a few rockets to space (to gather data packets for research).
With Spaced Out, in addition to water and dirt you will have to do some radiation-based research to get to the mid-game. A fourth kind of research, needed to complete the research tree, uses Data Banks. Some Data Banks can be found on your starting asteroid. To unlock the whole research tree you will need to get more, for which you will need to send some rockets to space.
Solar power (or: the wonders of life without meteor showers)
In the base game the challenge with getting to solar power isn't just being able to manufacture glass. The main issue is the frequent onslaught of meteors crashing down on the asteroid's surface. This is a problem that is entirely possible (though both laborious and tedious) to solve, but requires a little bit of engineering and a huge amount of steel, a late(ish)-game material.
In Spaced Out, it used to be that there was only one asteroid with meteors. As of the Whatta Blast update (March, 2023), there are now meteor showers on several asteroids. However, you can change the frequency of meteor showers (or disable them completely) in the setting. (Also, my understanding is that previous saves from before the update remain meteor shower free.)
Without meteor showers, mid-game power becomes much easier: as soon as you can get your hands on glass you can start tapping into solar power.
(There are new buildings to help defend against meteor showers. I haven't pleyed around with them enough to have an opinion.)
My personal opinion
Getting a break from meteor showers was one of my favourite things about the DLC. I'm not sure I'll learn to love their being part of my game again, but I'll give it a go.
But my personal hangups aside, I absolutely recommend getting the DLC. For the new geysers. For space travel. For a (potential) break from meteor showers. Or just to support Klei so they can keep developing new content.
Now let's dive into the actual game!