A. The difference is that one is indulgence in pleasure and the other practices self control. Both methods have the same effect, limiting family size, but that does not make them equal morally. For instance, if Grandma is terminally ill you could kill her or allow her to die naturally. The end result is the same but the moral difference could not be greater. Never do evil so that good may come.
The couple who practices NFP abstains from pleasure in order to limit family size while the contracepting couple indulges in pleasure stripped from its meaning and purpose.
The NFP couple practices total self giving love in their union but the contracepting couple rejects the fertility of their mate and so the union is incomplete.
God designed sex and eating to be pleasurable so that we would not forget to procreate and nourish our bodies. Stripping the pleasure of sex away from its God-given purpose is disordered just like attempting to get the pleasure out of eating but avoiding the natural purpose by throwing up after every meal.
God created the marital union to be a tri-unity of pleasure, bonding and openess to children. The attempt to extract pleasure and bypass the purpose of sex and eating is called lust and bulimia.