I'm not sure which one I read first, but whenever I think of either, it immediately reminds me of the other: The Count of Monte Cristo alongside The Gadfly. Many things can be said about these two, but it's simplest to say that while Monte Cristo is romantic, The Gadfly is much closer to the reality.
There are some similarities between these two: the protagonists are perfect and pure in the beginning, but soon they're cheated and isolated many years from their previous lives (love interests, families, belief sets).
That's when the two novels seperate ways: Dantès is blessed. He happens to inherit piles of gold and gems, he makes himself noble, goes home, gets sweet revenge and wins back the girl (who is now a lady). In short, he fixes things right. Meanwhile, the desolation twists and turns Arthur into this new man, who is almost a creature with violent tendency and deeply buried belief. He goes back home with scars not only on his face and arms, but also in his mind and heart, a mistress, new name, new appearance and above all, his transparent hatred and secret love. He is torn between the burning desire to believe again, and the despair the betrayal has brought him. He dies a death that no reader can ever forget.
I think while both novels are splendid, The Gadfly makes a much better reread than Monte Cristo due to its humane aspect.