How does someone know if they should hire a secretary or a personal assistant?

Agree! There is maybe a small difference within the duty. The personal Assistant has more obligations and more responsibilities in the organization of the day and the secretary is more table work including tipping, reminding, basic communication with costumers.

Now days, the difference is the same, Both a Secretary and a personal assistant do very similar functions

I think PA is more like executive secretary and more senior. Even PA can manage one or more secretaries. PA has more power and can make decision based on manager's delegation