I planned to go with the 19's had I proceeded with my order, primarily because I prefer the looks. In theory, the 18's should give you a more comfortable ride, but given the MKC's comfort-oriented suspension I'll be surprised if most people can even feel the difference. There is 1/2" less sidewall to protect the wheels when you step up to the 19" option, so that would be a bigger concern for me, and the main reason I wouldn't personally consider the 20's. (You may have heard ... we have a few potholes here in Michigan.) You might want to look at what tires are available in each size before you make a final decision. 19's aren't as common as 18's or 20's, so you may find better tire options if you stay away from the 19" option.