It's not about looks, it's about form following function. 18's perform better than 19's and 19's better than 20's. It's all about keeping as much of the weight of the wheel as close to the center of rotation as possible. As well as lowering the unsprung weight of the car. Also, In order to run a bigger rim, like a 20, and maintain your stock overall wheel height (tire OD) you would have to lower the aspect ratio of the tire, which means less sidewall. Less sidewall translates to a worse ride and in many cases and unpredictable loss of traction at high lateral loads. Personally an 18' is the best compromise of style, weight, performance, tire availability, and tire cost. Tire cost will be very important as with most cars I drive they won't make it much past 10K miles. I won't even consider a wheel if it's not under 19.5 LBS. That weight narrows the field considerably.
A good example is my old IS300, before I turboed it. I had HEAVY 19's on it (around 31lbs + tires). The 19's over the stock 18's raised my 0-60 by almost 2 full seconds. Later on down the line I replaced them with light weight Axis Reverbs. I ran Reverbs on my Mazdaspeed 6 too. They were a great rim but I just can't see buying the same wheel for three different cars.
And just a little footnote. I ran Axis Millano 19's on my 350Z and I was pleased with them. But knowing what I know now I would have gone with an 18 if I had it to do over again.
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