depends on what you want the planet to produce

for artifact production Rings is the bedst option, as you have a limited number off artifacts that give you a size 2 to 2 artifact prodction ratio

for mining and recearch, you have quite a few production artifact thet will give you a size 1 to 2 production or better ratio, if you fill your planet with those, you will usealy end up with higher production from the extra space you get from a moon then just the 20% bonus you get from rings

but as most players want theire special planets to produce artifacts, rings is usealy the bedst upgrade to get on special planet


see: viewtopic.php?f=3&t=32236

And here's where you're wrong. Rings will ALWAYS be better.

not True Silver_Stiched_Crow



and on this



rings = +20%
large moon = +14 spaces again, the simpel version is to presume +12 production from a Star Lifter, but need to take of 1 Obviation Barrier to install it
12 / 44 * 100 = +27.3 % bonus from getting a large moon

or I could add mining artifacts, dont have Cognizant Omniforge (Size:1, Artifact:1, Research:1, Mining:1 +5% bonus production), Heist Dispatch Station (size 2, mining 2, artifact 2), Orbital Stabilizer (size 1, artifact 2, mining bonus +14%), Litheor Core-Tunnel (size 1, mining 2) or (size 2, mining 3), Galactic Concord Station II (size 1 Mining: 2, Artifact: 2) or the 2 Deep-Phase Probe (Size 1, +2 Artifact/Research/Mining, +900 Pop, +150 Cloak.) or Extracted Lithovoric Stem (size 1, Bonus Mining: +10%) on planet

with the right artifacts buildings, that +14 spaces easily adds up to more, then you then you get from rings
as long as it is a single production planet, but if I stick to my plan and also boost it to x15 in artifact production, sure, I would prefere to get rings on it.....

Crow is right, if you do it right, rings will ALWAYS be better.
It's a choice between +20% production and +20% space. Assuming you fill with the best structures first (better than 1:1), that extra space will only be filled in with worse structures, say 1:1 ratio since all the better structures have already been placed (such as hypergates or litheor probes). This will clearly yield less than +20% production.
The only way moons are better is when you happen to be one or two spaces short on your planet, and the extra space allows to you get a whole extra structure on, for example a +24. This doesn't happen often, and will even less now that legion missions give many 2:1 production:size ratio structures.

you need to get structures such as hypergates, litheor probes or the new legion mission structures before you can place them on your planets.
so what it comes down to, is where or not you have enougth artifact structures to replace a Star Lifter / Hypernode Brain size 16 / production 12 vs how mutch space you use for cloakes/defence

and I wouldn't bother upgrading Star Lifter / Hypernode Brain to production 24, as you only get 20 Trellith Structural Energizers


so for me what it comes down to for recearch / mining is

the size 16, production 12 (+ 20%) 14.4/16 = 1 : 0.9 size to production ratio
vs
the size 1 production 2+ production ratio (usealy with bonuses to defence, cloak or other stats added)

sure, if you have high enougth artifact production, getting Hypergate, DM79 Datacenter, Isolation Bureau, Galactic Concord Station on all your god planets isnt a problem
until then, the basic setup you will find on most mining planet

1 Warp Gate / Hypergate (size 1 Mining: 1, Research: 1, Artifact: 1 )
1 Processing Core (size 1, Mining: 1, Artifact: 1)
1 Refining Lab (size 1, Mining: 1, Research: 1)
2 Phase Cutter Chassis ( size 1, mining 1, attack 100)
2-3 Star Lifters (size 16, Mining: 12)

= size 37, mining 29 (or size 53 mining 41 with 3 Star Lifters if you go for low defence option)

+ some artifacts, depending on what the player have acces to, planet type etc

1 Assimilation Locus ( size 2, Mining: 4, Population: 200) and can be upgraded to +100%
1 Sulcation Portal ( size 2, Mining +2, Population +200)
2 Tri-Phase Cutter (size 1, Mining: 3, Attack: 300)
1 Volcanic Heat Seeker (size 1, Mining: 2, Population: 150) if the planet is volcanic/demon
1 Mylarai Stripcore Mine (size 1, Mining: 4, artifacts -2)
1 Captured T.O. Harvester II Mining: 2, Artifact: 1, Population: 300
1 Heist Dispatch Station (size 2, Mining: 2, Artifact: 2, Attack: 300, Attack: 10%)
1 Isolation Bureau (size 2, Mining: 2, Research: 2, Artifact: 2, Cloak: 400)
1 Galactic Concord Station (size 1, Mining: 2, Artifact: 2, Defense: 20%)
1 Hypergate (size 1, Mining: 2, Research: 2, Artifact: 2 )
etc etc etc
(you got more artifacts and upgrades like these, from legion missions, NPC kills, mission chains and season missions/NPC's)

sure, if you have enougth artifacts like these you can replace the Star Lifters slowly with some off these artifacts, and increase production
but, some off them are limeted you only get 6, 10 or 15 from the mission chain, others cost artifacts or badges to build/buy or require you to kill hard to find NPC's
so in reality, you usealy end up with 3-4 off these artifacts in cargo giving you 6-10 production - not enougth to replace the 12 production you get from the Star Lifters
a moon alowes you to sudently add the artifacts you have saved up, without having to delete the Star Lifters and will it that case usealy give you more then the +20% you would get from rings

so depends on how developed your planet is, and what artifacts you have acces to
see a lot off planets like those 2, where you would benefit more from getting a large moon then you would from getting rings, but then again, I wouldnt bother buying a Reality Transfuser for a mining/recearch planet
as long as you have acces to blade havesters, you can always just make another Toxic Very Large Mining: 16x Mega Rich instead to increase your mining income

Rings i think.