grammar - When is "事" used instead of こと for past experiences?

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?

Answer

I believe those two (ことがある and 事がある) are the same construction in terms of meaning and they only differ by the way こと is spelled.

こと used as a grammatical construction, like the one in the question, is more often spelled using hiragana in modern Japanese.