The search function is your friend here. many opinions.
If you put on a 245/45-20 it is the same sidewall height as the 19's (4.34) . Will that fit the car with adequate clearance? The diameter difference is .98.I have 20's on my XTS Premium, yes, 19's have a slightly bigger sidewall, so slightly smoother ride. However those Bridgestone Potenzas don't do either the 19's or 20's justice. It really isn't a bad ride with the 20's. However when I put my Continental DWS tires on. Huge difference, smooth, not bumpy at all, excellent cornering, steering, low road feedback, and ect! The sidewall size on the tire overall on those 19s is 4.34, where on the 20's its 3.86. The total difference is .48 inches.
Yes, several big differences - ride comfort, vulnerability of rims, etc. Check previous posts on this forum for full discussionIs there a discernable difference in ride between the two, and if so how much?