When to use a Long Cage vs Short Cage Derailleur? Ask Speedub.Nate

Time to buy a new rear derailler (yes, derailler. I speak English, not French - i.e. "derailleur"). Stoopid rock! Wait, mountains are made of rock. Ok, stupid loose rock with a bad attitude. So do I need a long, medium or short cage Derailler? Well no better answer to that question than the one posted by Speedub.Nate on MTBR.com http://forums.mtbr.com/drivetrain-shifters-derailleurs-cranks/when-use-long-cage-vs-short-cage-derailleur-205890.html You da man, Speedub.Nate!