I've given mine both! I did it a few times at first so I could skip a breast feeding and pump to store up some milk. He gets formula about ever 3rd feeding, and it helps soo much at night to get him to sleep a little longer and save me from being frazzled! He has not shown any confusion on nipples and even takes a pacifier that is a little differnt shape.
Anyone else supplementing their LO with some formula?