On a mountain bike, your choice is slightly more difficult. Less experienced riders, or riders who ride frequently in steep terrain should probably stick to a long cage rear derailleur. This, combined with proper chain length, will allow the bike to be safely shifted into every gear on the bike. Racers might appreciate the lighter weight and slight shifting improvement a short cage rear derailleur offers. However, these benefits come with a tradeoff - because of the decreased capacity a short or medium cage rear derailleur offers, it may not be possible to safely use the small chainring in tandem with the smallest cogs. (The derailleur simply can't take up enough slack in the chain) Meaning if you typically have shifting issues, don't go to a medium cage cause its only going be exaggerated!