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It's been many times when it comes to decide whether to use "-or" or "-er" as an agent noun suffix to a verb. My native (mother language) is Greek, and I'll try to provide you a way to think before making a decision.
Although both suffixes are correct...but not so much...

Here's how it goes according to the meaning being passed to the reader!

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  USING THE "-or"

  
  
  

  The "-or" is used to either:

  
  
  
  
  
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    ASSIGN AN ABILITY to someone.

    
    
    

    For example we say:
    

    
    
    

    
    

    "He is the terminator"
    

    
    

    
    
    

    that emphasizes the ability of someone to destroy...

    
    
  
  
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    DEFINE that someone HAS ALREADY DONE what the verb (prefix) does and also GIVE him some rights over the product of that action.
    
    For example we say:
    

    
    
    

    
    

    "He is the implementor of the SQLite JDBC driver"

    
    

    
    
    

    that emphasizes the one who HAS IMPLEMENTED the SQLite JDBC driver. He has already done it. He has finished this project and that implementation is already being used by client code all over the world. Thus, this also gives him some owner rights over his own project..(his implementation).
    
    

    
    
  
  
  
  


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  USING THE "-er"

  
  
  

  Instead we would use the "-er" suffix to either:

  
  
  
  
  
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    EMPHASIZE THAT THE ACTION keeps going on (not finished yet) or being maintained at the time of speech:

    
    
    

    
    

    He's been assigned the SQLite JDBC driver implementation. He is the implementer of the SQLite JDBC driver.

    
    

    
    
  
  
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    DISTINGUISH the ACTOR OF THE ACTION DESCRIBED by that verb from the OWNER OF THE PRODUCT OF THAT ACTION. (in other words, between the one who creates something and the one who owns that same thing). This is being accomplished by using both -er & -or.

    
    
    

    For example, assume that someone sells his implementation to a company that does the distribution and also the kick start of the implementation & availability to client code all over the world...thus, we say that:

    
    
    

    
    

    this company (BrandM Corp.) is the "IMPLEMENTOR" of that driver...

    
    

    
    
    

    and we keep referring to the author as:

    
    
    

    
    

    the original author is the "IMPLEMENTER" (Mr. Someone) of that driver.

    
    

    
    
    

    This distinction helps us separate the real author of the implementation (the assignee) from the project manager or company (the company that is the owner of the implementation (the brand).