Sorry. There is no way you should play that nocturne. That is suicide. If you can play the Rachmaninov Prelude, then you have to assume that your opponents can too, and if they played the Alla Marcia and you played the nocturne, they would win. If you can play the prelude well, then you should definitely play it. That is a pretty difficult piece; a lot more difficult that the nocturne, and the judges will know that. Whether or not the piece is as pretty as the other doesn't really matter when you are playing to win. You should listen to some of the ugly crap that blows away competitions.
Corigliano Etude Fantasy comes to mind...
Ignore that. But seriously, the prelude is the way to go, absolutely.