Very good question which allows you to think through and compare the themes in these two Dickensian works. I am going to give you one response that will hopefully allow you to think through and come up with other comparisions you can use to answer this question.
For me, a key similarity is the theme of redemption and how both novels show that no matter how bad we are and no matter how long we have done bad things, we have the possibility to redeem ourselves and make amends (to some extent). Consider how both the main characters, Scrooge and Pip, show their capacity for making bad choices and how they hurt others. Pip, although he starts as an apparent innocent, quickly shows that his "Great Expectations" have corrupted him - the way he treats Jo and Bessie for example is shameful in the extreme. Scrooge, a more simple, "flatter" character is bad from the start. However, during the course of the stories, both characters realise that they have done wrong and make some moves to ammend their wrongs. Consider how Pip ends the tale a much wiser and reflective individual who is able to be honest about his faults and his mistakes. Scrooge likewise changes his character.
You might also want to think of how redemption is only achieved through some kind of suffering, how poverty is reflected in the novels and how wealth is viewed. Good luck!