names - Why does the Gemara sometimes use the word Ben and sometimes use the word Bar?

Why does the Talmud sometimes refer to someone as Ploni ben Ploni, and other times as Ploni bar Ploni?

Classic example:

In Bava Batra 14b the gemara says Rabbi Shimon ben Yochai

In Bar Ilan's edition of Yoma 26b the gemara says Rabbi Shimon bar Yochai