Both "Lamb to the Slaughter" and "The Gift of the Magi" have endings that are unique, creative, and definitely a distinct twist on the stories themselves. They both surprise the reader: in "The Gift of the Magi," we are surprised to learn that both Jim and Della sold their own precious possessions to show their love, which in turn rendered the gifts they bought useless. Though the loss of money is sad, the reader is touched by their love for one another. In "Lamb to the Slaughter," the reader is surprised by Mary Maloney's clever way of hiding the muder weapon, and by the fact that she is brazen enough to feed it to the police investigating her crime. Both endings also show resourcefulness and cleverness on the part of all of the characters.
Some differences between the effectiveness of the endings is in the emotions that they elicit. In "Lamb to the Slaughter," we are left feeling awed and slightly horrified by what has happened. In "The Gift of the Magi" however, we are left feeling sad but then touched and moved. "Lamb to the Slaughter" also has less of a surprise element to the ending; we knew she put the lamb in the oven. We just didn't know she would feed it to the cops. In "The Gift of the Magi," it isn't until the very end of the story that the surprise comes.
I hope that those thoughts help to get you started; good luck!
Thank you! This helps very much! :)