I think if you are doing this for show then go all out. "Go hard or go home" right? but seriously, 19s on a Z look humongous, esp w/ a nice drop. thats basically what all the show Zs run, but 20s in the back might look too big, you know what I mean? 1 inch is the difference from a beautiful Z to a ridiculous Z that everyone laughs at. I say just run the 19s for the show circuit and 18s for daily driving.