a knight or biship

When I play and I am in a stage were I can choose to take back with knight or biship and your opponent will take again.For me it`s quite simple, when my opponent has castled and I have a queen I like to keep my knight because he can mate in more places than biship but when I only have my rooks and pawns breaking free from both sides I keep my biships and use then for there long range abillaty to stop pawns