Choosing a RULE instead of a TRIGGER
This depends most likely on how your updates will be accomplished, because


  "
  A trigger is fired for any affected row once. A rule manipulates the query or generates an additional query. So if many rows are affected in one statement, a rule issuing one extra command is likely to be faster than a trigger that is called for every single row and must execute its operations many times. However, the trigger approach is conceptually far simpler than the rule approach, and is easier for novices to get right."


- Postgres Documentation

So it seems that you find  rule simpler is quite convenient in the scenario where you make a large amount of changes to the data. Either should work, though.