Why were both criminals that were crucified with Jesus abusing him in Mark, and not in Luke? Two thing stick out to me in the gospel accounts of Jesus on the Cross:

Mark 15:27-32
They crucified two rebels with him, one on his right and one on his left. Those who passed by hurled insults at him, shaking their heads and saying, “So! You who are going to destroy the temple and build it in three days, come down from the cross and save yourself!” In the same way the chief priests and the teachers of the law mocked him among themselves. “He saved others,” they said, “but he can’t save himself! Let this Messiah, this king of Israel, come down now from the cross, that we may see and believe.” Those crucified with him also heaped insults on him.
Luke 23:39-43 One of the criminals who hung there hurled insults at him: “Aren’t you the Messiah? Save yourself and us!” But the other criminal rebuked him. “Don’t you fear God,” he said, “since you are under the same sentence? We are punished justly, for we are getting what our deeds deserve. But this man has done nothing wrong.” Then he said, “Jesus, remember me when you come into your kingdom.” Jesus answered him, “Truly I tell you, today you will be with me in paradise.”


Were those that were crucified with him 'rebels' or 'criminals'?

Why does Mark mention that there were two rebels, both of whom were presumably insulting him &quot;Those crucified with him also heaped insults on him&quot;, whereas Luke has one criminal insulting him and the other recognizing his innocence and mission?
This potential discrepancy is addressed in Apologetics Press' question: Did Both Thieves Revile Christ?

Possible resolutions to the discrepancies between the accounts:


Possibility #1: Initially, both thieves reviled Christ, but then one of them repented. 



  After hearing Jesus’ words on the cross, and seeing His forgiving
  attitude, the one thief may have been driven to acknowledge that Jesus
  was indeed the Messiah. How many times have we made a statement about
  someone or something, but then retracted the statement only a short
  while later after receiving more information?



Possibility #2: The discrepancy could be due to a figure of speech called a synecdoche


Merriam-Webster definition: a figure of speech by which a part is put for the whole (as fifty sail for fifty ships), the whole for a part (as society for high society)…or the name of the material for the thing made (as boards for stage)
Note that the doctrine of plenary inspiration does allow for figures of speech, even as it applies to inerrancy.



Also, from the article in the portion relating to the possibility that this is a synecdoche:


  It is feasible that Matthew and Mark were using the plural in place of
  the singular in their accounts of the thieves reviling Christ on the
  cross. Lest you think that such might be an isolated case, notice two
  other places in Scripture where the same form of synecdoche is used.
  
  
  Genesis 8:4 indicates that Noah’s ark rested “on the mountains of Ararat.” Question: Did the ark rest on one of the mountains of Ararat,
  or did it rest on all of them at the same time? Although the ark was a
  huge vessel, it obviously did not rest on the many mountains of
  Ararat; rather, it rested on one.
  In Genesis 21:7 Sarah asked, “Who would have said to Abraham that Sarah would nurse children? For I have borne him a son in his old
  age.” Anyone who knows much about the Bible will remember that Sarah
  had but one child. In certain contexts, however, one might use a
  synecdoche and speak of one child (as did Sarah) by using the word
  children.