Ambience and ambiance are different spellings of the same word, referring to the special atmosphere or mood of a particular environment. While some dictionaries list ambiance as the standard spelling, ambience is far more common in all main 21st-century varieties of English. It’s worth noting, though, that ambiance tends to take precedence in contexts relating to art and design, but this is by no means a rule, and exceptions abound. Ambiance is the French word from which the English one derives, and ambience is an Anglicization. But in fact, the Anglicized word has been in English longer and was established long before the French spelling entered English as a vogue word in the 20th century. So the fact that ambience is more common makes sense, as it has been an English word longer.