You pose a false alternative.  God desires both.  He certainly desires that we show mercy, but not to the point where it mocks justice.  He certainly desires sacrifice.  That's how bunting came into being.  Baseball, he most theological of games, saw Divine virtue in giving up something for a greater good.  Bunting caught on, so now we have volunteers at soup kitchens, people sacrificing their hard-won leisure to help others; young men and women going into the military to serve their country; husbands giving up other women to be faithful to their wives; people giving up their own desires to do God's will.   All are sacrifices,  All are desired by God.