In the "ことがあります" construction about past experiences, when is "事" used, and when is "こと" used?
The textbook used in class uses "こと":
六本木の おんせんに 行った ことがあります。
If it wanted to use "事" instead of "こと", it could have, as that kanji was taught in the same lesson. (Lesson 5 of "Japanese for Busy People II", revised 3rd edition)
However, Wiktionary says that the kanji form can be used, and "事があります" gets some hits in the Tanaka corpus on jisho.org
When is "事" used, and when is "こと" used?