I think there is no correct answer to this question.
The biome itself should be better, in total the bonuses should be better, but you get the uber biomes from perks much earlier. Even if you rush orion, you need to research gaia transformation first.
So i guess uber gaia is better in lategame, if you get that far. Don't forget you never know where orion will be...

Why is there no option to convert Uber planets to Uber Gaia? I guess it makes sense fo Inferno planets (although I'd like an option to unf**k them too after I capture them from enemy Silicoid) but what about Forst\Grass?
With the level of technology that allows to build and terraform planets from asteroids I don't think it makes sense to prohibit Uber Gaia transformation.

Well, technically, you could unf**k ANY colonized planet with a Stellar Converter, if you don't mind being at war with the entire galaxy. Blow it up, then rebuild the resulting asteroid field.

I've never actually tried it, but I think it could work if, once you capture an Uber planet, you sell of its Uber building(s), if any, and all pollution control buildings.

If you keep the pop in Production, the resulting pollution should, in theory, quickly degrade the biome back down to a level where you could Uber Gaia it afterward (obviously after rebuilding the pollution control buildings)

As to which one is "best", I mostly agree with Sensei. The Uber Gaia's bulding gives the most bonuses, but those mostly seem geared to war production, which may not be useful if you're going for other victory conditions.

Personally, I like Forest with the extra pop AND pollution control - the pollution control helps offset a bit the pollution generated not only by the extra pop, but also the extra pollution you get from specials provided in 5X and DSMP.

Yep, pollution degade would work but it's such a pain.. It effects food production and you have to manage population which descreases production and slows terraforming waay down.

Basically it would be super nice if there was just an option to upgrade to Uber Gaia, ideally in a way that it wouldn't be picked up by auto-build and screw up your prefered bonuses for Forest.

As for Stellar Converter.. I don't think it works on your own collonies or uninhabited planets . At least I'm pretty sure it don't but the last time I tried I didn't have the mod.

Sure, the assumption about Stellar Converter is that it was going to be used on enemy colonies. You're correct that it doesn't work on your own colonies (although if you were to "gift" some to an enemy, well, who could be responsible for what happened to them afterward, hehe) or on uninhabited (but then again, those would be eligible for normal upgrading).

Interesting to hear the degrading might work on Ubers - I might actually try that, especially on Inferno (if I can generate enough pollution somehow).

Spud is working on, or thinking about working on, an "instant Uber", like the Instant Gaia at the end of the normal tech tree, but I'm afraid at this time Instant Gaia > Uber Gaia is the fastest way to get there after degrading.