As for looks? well that kit for a REAL skyline looks magic in every way, but the kit in game is less sightly because it has slight overhangs on the skirts (look at the rear quarter in-game view on this site's homepage) that give it more of a Japanese tuner look about it - whereas the real tuned one could almost be classified as having a European look.
Having not seen the real version of the Amuse Z (with that body kit), I'm basing my decision on the in game models and not the real ones. And I going to have to go with the Z - it just looks so great - it's only major flaw is that the guages don't work on the A pillar. Though if it was real v. real, then it might have a different outcome.