I see that both are used to translate "terraforming":
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.
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.