Both contain nearly nothing from their later masterpieces, but Corbucci is here the better action director and his peplums have a better flow than Leone's.
I disagree about Leone. I think there are many touches of irony and some great imagery in Colossus that do show signs of what was to come for Leone. Compare Colossus with The Last Days of Pompeii which, albeit directed by Leone as a favor, is not in the same league as Colossus. I do agree that Romulus & Remus shows very little of what Corbucci would later achieve.