恩 is all the other kinds of love between a husband and wife, e.g. respect for one another, caring for each other, grateful to have found each other etc.
愛 is the sexual love between them.
In Classical Chinese, especially poetry, seldom do you see 愛 as it is considered crude. 恩 is the word of choice. In the imperial days, when a eunuch marries to have companionship, this relationship is called 對食 i.e. 有恩無愛