We have all seen how the space gray iPhone tends to show signs of aging sooner than the silver model. Do you think it will be the same with the rMacBook? Will the silver rMacBook look better when it gets older?
Apple built, and was slated to release, a black MacBook Air years ago but Jobs himself demanded that they be the silver finish that we've all come to know and love because wear testing showed that it handled scratches and rubbing much better than the darker colors they were experimenting with at the time.
Short answer: Standard silver/grey will wear better over time and look "new" longer. Darker colors will develop a "patina" and show their wear more. Up to you what you think is cooler. They both function the same