One way of comparing these two works is to think about the way in which Morrison presents slavery and race in them both. They are key themes of both works, but what is interesting is the way in which Morrison varies her treatment of slavery and race in either book to achieve a different effect.
In Beloved, for example, the prime focus is on the dehumanising effects of slavery. Much is made by the schoolteacher of the "animal characteristics" of slaves, as he regards them as just another form of animal. He has so much success that he manages to convince Sethe that slaves are another animal form, much to Paul D.'s disgust, and he is forced to tell her that "You got two feet, not four" in order to correct her. However, this novel also looks at the way in which slavery not just dehumanises slaves, but also dehumanises whites through the monsters that they become. Consider what Baby Suggs says about whites:
...they could prowl at will, change from one mind to another, and even when they thought they were behaving, it was a far cry from what real humans did.
Slavery is shown to be damaging to all of society, whatever their skin colour, as it is an institution that prevents humans from being human.
By contrast, although the dehumanising of slavery is something that is very much in evidence in A Mercy, its key focus seems to be the way in which Morrison examines the different grades or levels of slavery and freedom experienced by a wide range of servants. The servants in the Vaark household seem carefully chosen to represent a deliberate range of different types of slavery. On the one hand you have individuals such as the blacksmith, who, although he is African, has never experienced being a slave. On the other extreme you have Florens, who is black and a slave. In the middle you have the "mongrelised" Sorrow, white indentured servants and a Native American slave. Morrison deliberately challenges our ideas of slavery to present us with a range of characters who experience a range of different forms of slavery.