golang：返回指针或传递引用

What is the best way to "build" an object.

Leme write some code:

type Car struct {
   Wheels int
   Doors  int
} 

This cars are stored somewhere, somehow. So should my interface be the type of

func (s Store) GetCar() *Car

or should I make it

func (s Store) GetCar(*Car)

and pass a reference to a variable?

Im looking for some sort of rule of thumb.

Thanks!