Is there a difference between using "of" and "for" as the preposition for "repent"?

The difference is subtle, and while "repent for" is becoming more common simply because it sounds more consistent with other phrases, that is a shame since "repent of" conveys something meaningful that "repent for" does not.

"Repent for" indicates a simple cause-and-effect relationship, e.g. "I committed some sins, now I have to repent for committing them." Just like, "I got a speeding ticket, and now I have to pay a fine for speeding."

"Repent of" has a more subtle meaning. It's closer to "repent about", but not quite: The idea is that you are attempting to take something within yourself - a part of yourself - and reform it, or distance yourself from it. Consider it a combination of "meditate on", and "dispose of".

That's a pretty different - and I'd say a more powerful - idea than simply facing consequences for an act.