You're right, in all the test I have seen, the differences are negligible between 17, 18, 19, 20. While wet and dry handling are on the opposite ends. That’s because the more rubber that comes in contact with the road, the better control you’ll have (on dry road). On snow, the narrower may have better "bite" although the wider ones are better on a hard packed snow. The less rubber, the quieter your ride would be. Speed wise, the bigger tires cover more ground so you probably always will be driving 2 to 3 miles above indicated speed.From all that I heard in talking to the dealer and reading online, it largely comes down to appearance. One more inch (19 to 20) is not likely to improve handling or create problems in winter driving, although some may disagree. In the end I opted to save the $800 and go with the 19s.