In general, running two sets is going to be better than just running one set all year round. However, I'm of the opinion there are some conditions where an all season compound could be better than either a summer or winter compound, especially if the tires have the appropriate tread pattern. All season tires feel "just right" in 40-50F, for example. That's a little warm for a lot of snow tires and it's a little cool for many summer tires. So regardless of whether you run one set or two, you are making a compromise in two of the four seasons. Perhaps the best approach, although highly impractical, would be to run THREE sets as one poster suggested above! Summer, Winter, Spring/Fall