The Bible is silent as to which of these individuals expressed his faith so eloquently to the Lord. See Luke 23:32-43.
By the way, it is an assumption not warranted by the text to assume they were thieves. The word in Greek means simply a "criminal". They could have been murderers or forgers or perhaps guilty of sedition against Rome. In the Septuagint [Greek version of the Old Testament, this word is translated as "workers of iniquity" in Proverbs 21:15]
In a sense, the two wretches together represent humanity - the world of hopeless unbelief and that of strong conviction with Jesus in the middle, all sharing the same experience of terrible suffering and, finally, death. Only Jesus and one of the "malefactors" would survive. Which one of these two criminals represents you?