Back in my road racing days (mid-late 80s), all road racers road short cage, and long cage was for touring bikes and mountain bikes. Realistically, long cage vs. short cage just determines how much extra chain the derailleur can take up to maintain tension on the chain. Longer cages accommodate a wider gear range better, giving you a more stable chain line and reduced slack. A short cage derailleur weighs a few grams less than a long cage. Some believe that short cage derailleurs have faster, snappier shifting than long cage derailleurs; not sure if there is proof of this. Both may actually shift at the same speed. Short cage derailleurs look more sporty, though.