Don't take the easy way out; don't love the child who is easy to love and ignore (or even dislike) the one who is less lovable. You not only will damage them, but you will bring great harm to your family.
Do you remember the story of Isaac and his wife, Rebekah, in the Bible? (You can read about them and their family in Genesis 25-27.) Isaac and Rebekah had twin sons, Jacob and Esau. The problem was that Isaac loved Esau more than he did Jacob, while Rebekah loved Jacob more than she did Esau. The result was almost constant conflict conflict that could have been prevented if they had acted more wisely.
Begin by realizing that God gave both of your children to you, and God loves both of them equally. Thank God for both of them, and ask Him to help you love them the same way He loves them. In other words, ask Him to help you see both of your children through His loving eyes.
Yes, one may take more effort than the other, but God will reward you for your faithfulness. After all, aren't we often just like difficult children in the way we treat God? But God still loves us, and "Christ suffered for you, leaving you an example, that you should follow in his steps" (1 Peter 2:21).