According to Google Translate it is "Strange" and "Strangeness"
But I saw a native speaker post "好诡异", which would be "how strangeness?"
When would you use one or the other?
Both 诡异 and 奇诡 are adjectives. The "strangeness" translation is incorrect.
诡异 is a commonly used word meaning "strange". It can be used both colloquially and in writing. The idiom "好诡异" you mentioned means "How strange it is!".
奇诡 is a formal word used exclusively in literatures. It is a stronger form of 诡异 and is usually reserved for things that are extremely bizarre or supernatural. Use this word with caution.
Both "诡异" and "奇诡" means strange or something out of the the ordinary, its a move elaborate expression than "奇怪", "奇异", "怪异"。
"好诡异" can translate to "how strange" or even better "so out of the ordinary".