I went with 19s and love them. Plenty of lip and no lack of performance. Some wheels like TSW Thruxton (sick by the way) offer 19s and 20s in the same style. I have seen some Z owners in my area do both with 20s rear and 19s front. I couldn’t say what that does for performance but it looks O.K. I think 20s in the front puts the rims in jeopardy. Too many pot holes out there.

 

i have both 19;s and 20;s. honestly i could barely tell the difference between the 2. BUT with the 19;s it handles a LITTLE better and the performance improved slightly. But if you ask me the 20's look the best ONLY with a body kit though. IMO if you get 20's plan on dropping it with hotchkis or whatever andpurchasing a full bodykit. The Z with 20s minus a bodykit doesnt look bad but I think it looks better with a body kit. It flows a lot better. I had the 20s on stock and it was nice but they looked so huge, then i put the nismo kit and dropped with hotckis and looked much better. Back to the main topic on performace. Its not really a huge difference, but like everyone else is saying dont plan on tracking with dubs on..

 

+19s FTW!

 

Both of them are not for performance anyway.

+1 for 20s

 

I would stick with 19' bro. Since you just said roads in san jose are horrible, do you think that 30 series tires would take the punishment? I'm sure you already know the answer to your question. At least you have a little more cushion with 35 series tires on 19's oppose to 30 series tires on 20's. believe me, my friends who have 30 series tires always bend their rims or had a flat with 30 series tires.
Go with 20's if you want to look bling. But you will eventually pay the orice for having a low profile tire. Go with 19's for performance and looks.

 

If you're going to get 20", don't use Toyo tire. They are too soft for 30 profile. I use BGF 285x30x20 and they last more than 15,000 miles now.

 

I have 35 series on 20's both front and rear. It doesn't have to be 30.

 

It doesn't have to be 30 series, but majority of the 20' wheel owners have 30 series.