stock 18's ride better, 19's look better and will 20's fit without doing any mods?? i have 07 is350 by the way, stock!!

 

all those sizes would fit fine without any fender modification depending on the offsets of the wheels and size tires you run. 18's would ride better because you would be running 40 profile tires comparing to 19's where you would have to go with 35 profile tires. The lower the profile the more uncomfortable the ride would be because of having less meat on the tires. In my opinion I think 19's are perfect for the ISx50's

 

if it was only so easy to judge ride quality on wheel diameter and tire sizes... but it's not.

while tire sidewalls absorb a good amount of road vibration, it's a small amount compared to what the rest of your suspension (and the 2is chassis) is doing to create that ride quality. you can run larger-diameter wheels and lower-profile tires, but if you are able to compensate that with softer spring rates and properly-tuned shocks your ride quality won't suffer as much.

it also depends on what brand/model of tires you choose. two different types of tires from the same manufacturer can have completely different handling/ride quality characteristics.

if anything, ease of fitment correlates to wheel diameter: the difficulty of getting the proper wheel fitment for your driving needs increases as wheel diameter increases. or, bigger the wheel, smaller the window of error.

thanks for reading.