Nothing is better than the other. They are both of the same calibre and have their own stories to tell.
Dark Impetus is crazy frantic, which goes well with the nature of the fight; you fight against someone whom you don't know, was constantly in almost-God mode, and whose one hit could bring you down to whittling HP (seriously, it used to be hard to imagine that sound without the critical HP siren). L'Impeto Oscuro worked as a slightly, albeit ever so, toned down arrangement because you already know whom you were faced against, but it still retains the urgency of Dark Impetus which is appropriate considering Young Xehanort was the main final boss of the story.
The most important aspect is of course the violin solo, which is sort of like the "identity", the "heart" of the pieces. Once L'Impeto Oscuro reaches that point, you'll piece the puzzle together and be brought into a mind-blowing realisation that the Mysterious Figure and Young Xehanort are nothing more but the same person.