According to the French version of the wiktionary, “Terraformation” is the science behind the terraforming. “Terraformage” is more like a completed process of terraforming, a final result.

As a proof, we can see at the end of “Terraformage”, coming from the verb “Terraformer”, the word “-age” is often used to speak about the result of an action.

I agree with what has been said before, but I think there is no correct answer yet.

They are new words, both not in dictionaries (*) yet so you should be careful if you use them in a paper. They’re both attempts of translating “terraforming” and I’d say they mean the same thing, only time will tell if they’re synonyms, variants or if one of them is just wrong.

(*) Wiktionnary is a community-based dictionary. It’s very good in most cases but be careful with neologisms.