Oracle Triggers – Separate or Combine Before Update and Before Insert

I'm running Oracle DB 9i.

I have a table with various constraints to ensure data integrity. In addition to the constraints, I have triggers on BEFORE INSERT and BEFORE UPDATE to ensure that necessary data goes into the table, in some cases allowing the application layer to omit the information from their queries, and in some cases forcing that it be present.

The actions that I need to do on my BEFORE INSERT are different than my BEFORE UPDATE.

My question is this:

Can I have one common trigger that uses IF / THEN, or should I make separate triggers?

For example:

-- Scenario 1
   CREATE OR REPLACE TRIGGER   my_trg
              BEFORE INSERT OR UPDATE ON my_table
                 FOR EACH ROW
   BEGIN
      IF UPDATING THEN
         IF :NEW.my_col IS NULL THEN
            raise_application_error(...);
         END IF;
         IF :NEW.my_col2 IS NULL THEN
            :NEW.my_col2 := (...);
         END IF;
      END IF;
      IF INSERTING THEN
         IF :NEW.my_col3 IS NULL THEN
            SELECT my_seq.NEXTVAL INTO :NEW.my_col3 FROM DUAL;
         END IF;
         IF :NEW.my_col4 IS NULL THEN
            (...)
         END IF;
      END IF;
   END;
/

Or:

-- Scenario 2
   CREATE OR REPLACE TRIGGER   my_trg1
              BEFORE INSERT ON my_table
                 FOR EACH ROW
   BEGIN
      IF :NEW.my_col3 IS NULL THEN
         SELECT my_seq.NEXTVAL INTO :NEW.my_col3 FROM DUAL;
      END IF;
      IF :NEW.my_col4 IS NULL THEN
         (...)
      END IF;
   END;
/

CREATE OR REPLACE TRIGGER   my_trg2
              BEFORE UPDATE ON my_table
                 FOR EACH ROW
   BEGIN
      IF :NEW.my_col IS NULL THEN
         raise_application_error(...);
      END IF;
      IF :NEW.my_col2 IS NULL THEN
         :NEW.my_col2 := (...);
      END IF;
   END;
/

Currently, these two triggers are combined. Should I separate them out?