One reason that can be purported for the worthiness of the first ending as opposed to the latter one is the fact that the original ending possesses an integrity that the latter does not because Dickens felt pressured by his readers to rewrite the conclusion.
Willie Collins, a close friend of Charles Dickens and author of The Woman in White, objected to the first ending in which Pip remains single while Estella remarries. Dickens then wrote a more conventional ending in which Estella and Pip may marry, saying that he thought the narration would be more acceptable as a result. However, another of Dickens's friends and his biographer, John Forester, felt that the original ending was
more consistent with the draft, as well as the natural working out of the tale.
English writers such as George Bernard Shaw, George Orwell, William Dean Howells and others of the 19th century and early 20th century agree with Forester, feeling that the original ending is more in character with the heartless Estella who is incapable of remorse and of reciprocating the kind of love that Pip feels for her. However, more contemporary criticisms argue for the second, more maudlin ending probably because efforts in modern society are constantly being made to keep people happy and satisfied.
Of course, this is largely an issue of taste. The original ending is more lonely and bleak whereas the revised version gives some amount of hope that Pip and Estella will one day get together.
People who prefer the original ending generally think that the revised one is just a bit too sappy. They feel like the book as a whole is not really meant to have that kind of a happy ending. Therefore, the say, the revised ending seems forced and doesn't really fit with the mood of the book as a whole.
If you want to look at some arguments each way, follow the link.