depends on the Planet.....
viewtopic.php?f=3&t=32236
on a mission Dyson, Rift or exotic rings are better, as what you want from these is realy artifact production
but on a standart Mining or Recearch planet you can squese more production out off the extra space you get from a large moon, by adding small production artifacts, compaired to just getting +20% from the rings
on a planet like this one
rings would give me +20% bonus
a large moon = +15 spaces would more or less give me 1 extra Hypernode Brain = +12 production (more if i find some artifacts to fill the space with)
12 / 49 * 100% = 24.5 % increase in recearch production
what it comes down to is
Hyperforge Fabricator (100% Upgrade) (size 16, Artifact: 24) and Hyperforge Fabricator (size 16, Artifact: 12)
vs
artifacts like, Galactic Concord Station II (size 1 Mining: 2, Artifact: 2, Defense: 20%), Silthion Gas Vesicle (size 1, Research: 2, Cloak: 150), Tri-Phase Cutter (size 1, Mining: 3, Attack: 300), etc etc etc
for mining and recearch, you can easily find artifacts that give you a 1 : 2 size to production ratio or better
the production 12 structures are "only" 1 : 1.5 / 1 : 0.75 (or 1 : 1.8 / 1 : 0.9 if you include the bonus you get from the rings)
but for artifact production, you dont have as many small production artifacts as you do for mining and recearch, so on your artifact planets you want the +20% bonus instead to max the production from every structure you put on it
when it comes to Advanced Civilization Ruins, again it depends on what you want the planet to pruduce
on a plasma planet, they would basickly be worthless, unless you took the time to booost artifact production to x15 with the Taltherian ability....
but if what you want, is artifact production they are almost as good as rings
only difrence betwen them is, rings also boost mining and recearch +20%
once you have 10.000+ Scientists on ship, and 10+ Toxic Very Large x15-16 Mega Rich / Irradiated Massive Mining: 17-18x Mega Rich, all that realy matters to you is the increase in artifact production