17 inch VS 18 inch tires and wheels
 

Some dealers are telling us that the 17s are smoother riding than the 18s on the TL. Some are saying that there is little difference. What say you out there with your cars?

i dont have a 4th gen but my 18's definitely ride a little harsher than my 17's ...

The larger diameter the wheel the rougher the ride. In other words 15's will give you a smoother ride than 17's.The more sidewall tire the more comfortable the ride.
The key is compromise. The OEM wheels available on the 4G are 17's , 18's & 19's. I just don't like the look of the 17" wheels, so I opted for the 18's. The 19's look awesome, but I am willing to compromise the looks of the 19's for the greater comfort of the 18's.

Like what jspagna1 said, its about compromise. I will just add that its also about priorities. I wish I could have gotten the 18/19's because they look awesome! During my test drive, the 18s definitely rode harsher than the 17s. I test drove through a rough street and my friend sitting in the back felt like she was bouncing around. I went with the 17s because of cost, softer ride and I do chauffeur my older relatives around quite often and I don't want them to have what they would consider a jarring ride.