In some sense the second option is the explicit counter-factual form, where just "If I were to [perfect infinitive]" alone is enough to convey that the condition never occurred. The first option means the same thing because of the subjunctive "would [perfect infinitive]" in the conclusion of the sentence. There is also the implicit assumption that a person would know whether or not he had left a job, and hence would only say "If I had left this job" if he was stating a counter-factual. However, consider the following scenario where there is a difference:
If I had offended you the last time, please forgive me.
In this case the speaker/writer implies that he/she does not know whether he/she had offended the other party or not. And indeed for that same reason the counter-factual subjunctive in the condition would be incorrect:
(WRONG) If I were to have offended you the last time, ...
However, the present subjunctive can be used for a possibility of a choice that might be taken:
If you help me, I would be ever so grateful.
If you were to help me, I would be ever so grateful.
But the present subjunctive should not be used in a conditional sentence to denote something that cannot be chosen:
If I make an accidental mistake, please let me know.
(WRONG) If I were to make an accidental mistake, please let me know.