Erin Cements the Role
Even before the girls stopped looking identical, it was clear that one child cooperated more than the other for filming purposes. Erin Murphy was better for the purposes of the show, so was used for any close-ups and speaking roles, while Diane was filmed for long shots.
While both might have a spot on IMDB for their role as Tabitha, Erin Murphy was given greater credit as the child star and has appeared in interviews and TV talking and reminiscing about her childhood stardom. However, both girls were able to grow up far enough away from the Hollywood spotlight.