I didn't think the X6 was less comfortable at all. Thought they were comparable given the same size tires. I had ordered an X5 with 19" and then cancelled it in favor of the x6 with 20's. The sportiness and handling were the key advantages in my mind. I had much more fun and less body roll on the back roads with the X6 and didn't notice a ride penalty.

Great choice to have so you cannot go wrong. If you are worried about the ride though, the X5 with 18"s is the safest way to go but would look terrible on the X6. The 19"s are the sweet spot on the X5.