My all-season seasons are on 20-inch rims, and my snows are on 18-inch rims. I cannot say I have noticed any appreciable difference in the ride except possibly just a little bit quieter with the 18's.1) No
They aren't 20" tires. They're 20" wheels. The overall diameter of the tire is the same - it just has a smaller sidewall than a comparable tire for the 18" wheel.
2) Only a little.
A tire is part of your total suspension system. There's "give" to the sidewall in a tire. A tire with a smaller sidewall will have less give than one with a bigger sidewall. Therefore, a QX60 with 20" wheels will ride very marginally firmer than 18". You'll barely notice the difference.
I would agree, we went with the 20" since we thought they looked more proportional to the car.What do you think you are going to go with in the end? The 20" ones probably cost more and are considered fancier. I think it mostly comes down to looks and not driving dynamics.