From a novice to those in the know....What darn difference does it make if I have 18", or 19", or whatever size wheels? If I have to pick a size for my M3, it might be beneficial if I knew what dang difference does it make. All I know is that they all roll down the highway. I don't have a clue as to why I should buy one over the other. Is it purely appearance or is there a technical reason? I hope I'm not alone in this lack of knowledge. If you know, would you share the whys?