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.