Entity reference preformance : one to many or many to one

I have two content types : Cart and Cart line item. I need to link them by using entity reference. One cart may have many line item and one line item belongs to only one cart. In term of performance (query, views,...), which approach is the best ?

• Create one multiple entity reference field on cart and filter by content type line item.
• Create one unique entity reference field on line item content type that refer the corresponding cart.

Best.