Vanyel
Stars are a representation of your driver's level compared to yours.
Driver talent isn't something that can be trained so it's best to try and grab yourself a driver with as high of talent rating as you possibly can.
Homegrown drivers, as you call them, will increase in talent as your Academy increases in level. You will eventually find young drivers with 20 talent there. The advantage of grabbing a driver from your Academy is that they are young and don't demand a huge salary so doing so saves you money.
Edit: To answer your main question, talent is more important than stars.
Perfect answer