Both are centered around a legendary masked heroic thief, who fights the rich and helps the poor. They are driven by revenge and a distinct sense of justice. They also have exceptionally good fighting skills. Also both involve an interesting father/son relationship as well as romance (Rebel has better romance than Iljimae imo).
However, I'd dare to say that Iljimae is a lighter watch than Rebel. Latter is more complex and involves more characters.