If you are serious about making the comparison, you really need to read "The Coral Island." You can get decent, slightly abridged editions; the original is something of a tome. Also, I recommend the essay, "The Coral Island Revisited" by Carl Niemeyer, published in College English, 22 (January, 1961), and in the Casebook Edition of "Lord of the Flies" (Perigree Books: New York, 1988)
As you will see, "The Coral Island" is very pro-British, very Christian, and very moral. Golding has admitted that LOTF is something of a reply to this Victorian view of the world, in which all adversities can be overcome with hard work and a positive attitude. But in forming your thesis you need to address the complexities of Golding's novel, which are legion compared with Ballentyne's.