If you were to only put 1:1 structures, then they're the same, but rings are better in general. For moons to be equal to rings, you need to generate 20% more from your structures in that extra 20% of space, which is incredibly difficult to do because it will require a lot of limited artifacts. If you generate under than 20% more from your extra space, then rings are better. I prefer rings because you do not need to use a ton of artifacts to fill up the space and you will end up with the same production anyway, assuming that situation of having the same space efficiency across the board.
Anyway, I'm going to go out on a limb and say you'll probably have a couple structures here and there that have a greater than 1 artifact per space used that you will want to put on it. Maybe a concord station, trellith relay, a manufacturing core, you name it. There are a small number of artifacts that have a greater than 1:1 size to artifact ratio. You will likely use these on the standard space, and a few 1:1 sized artifacts. Let's say your ratio of artifact produced vs space used is >1 on a standard sized planet. If you get 20% more space, you will likely not have any >1 artifact/space artifacts left due to already using them on the standard space. You will likely only get a ratio of 1:1 for that extra 20% space, in a best case scenario. You will end up only increasing your production by maybe 10 or 15%, whereas rings gives 20%. So best case scenario is you match rings and get that 20% more, but use a ton of artifacts to do so, or have rings and be much more efficient with your artifact usage.
You should also note that due to multipliers, those few percentage points will make a significant difference. The bonuses from trellith, klorvis, hoveroid, cognizant forge, tradepost intelligence and litheor transmutative is 2.05401924. If you multiply 20% from rings, it's 2.464823088. So that 20% from rings increases to about 40% due to other multipliers. If you have moons and get say 15% more from structures, then that number turns to 2.362122126, a difference of about 10% compared to rings. So get rings if at all possible
