I have always seen "facilis descencus averno" as the translation for "the descent to hell is easy", but I saw it written as "descensus averno facilis est" and I'm in doubt now. Which one is correct?
This answer may be longer than you expected, but there's a bit to unpack here.
The translation actually goes the other way. "Facilis descensus Averno" is from line 126 of book 6 of the Aeneid. Either Latin phrase would be an OK (but not ideal) translation back of the English, but "Averno" would be slightly unusual for "path to hell" absent the context of Aeneid 6 (where Aeneas is entering the underworld via a cave near Lake Avernus): And note that Lake Avernus isn't literally hell (then it would be in the accusative as a destination) but [part of] the path to it (descent via Averno).
In a more neutral context "facilis descensus ad infernum" or "facilis est descensus ad infernum" would be the Latin translation of the English phrase. You would probably want "facilis" to be the first word. Latin word order is relatively free, but this means that if you have a word you particularly want to emphasize, you put it first, and in this case you surely want to emphasize "easy". Also, note that while in English we tend to associate "infernal" with fire and brimstone, in Latin the reference is to being underground--"infernus" simply means "underworld" much like the Greek word "Hades" does, but without the personification. It is also the word St. Jerome probably most often used for "hell" when he translated the Bible into Latin, though he occasionally also imported non-Latin words like "gehenna" or "tartarus" to mean hell (presumably simply pulling the words through without translating, but since I don't know Hebrew, Aramaic, or Greek, I don't know for sure).
By the way, Lake Avernus is a real place in Italy.
To sum up, if you are just trying to translate the English phrase into Latin, either "facilis descensus ad infernum" or "facilis est descensus ad infernum" would be fine. If you want to retain the reference to the Aeneid, then you should use Vergil's actual words "facilis descensus Averno"; it would be silly to change them to "descensus averno facilis est", since the only reason to say "Averno" is to allude to the Aeneid. Changing Vergil's words around would be like saying "This is the question: To be or not to be" rather than "To be or not to be; that is the question".
Late edit: There are variations on infernum that would be reasonable alternatives. For example, one might use the plural "facilis descensus ad inferna", since "lower places" was idiomatic in Latin similarly to "heavens" in English. If one wanted instead to emphasize the residents of said location, one would say "facilis descensus ad inferos" (easy the descent to the damned). "Ad inferos" is actually the word choice in the Apostles' Creed for "[He descended] into hell".
Finally, for those of you wondering why infernus (masc) has inferna (neut) as a plural, remember that this is an adjective used as a noun. In this case, it's the singular masculine that is somewhat exceptional; normally it would mean "lower man" but it is idiomatic that it happens to mean "lower place". (Maybe this is influenced by the Greek "Hades".) This sometimes also happens with nouns that are not adjectives that are masculine in the singular and are (or can be) neuter in the plural, like locus. (And this raises the possibility that infernus is a shortening of locus infernus.) In any event, the distinction is not very noticeable here since the accusative singular is infernum regardless of whether it's masculine or neuter. But I thought those who have been following the "neuter = collective in PIE" threads might appreciate this example.