Are you running Campy? Up to 29 teeth is fine on Campy short cage rear derailleur according to the parts distributor from the shop I worked at. A medium or long cage suits a triple crank. If you cross chain often from your 50 to your 25 you may want a medium cage. It gives you a little more chain to work with. (you will need a new chain if you go medium cage) and this will be better for the longevity of your derailleur and keep the stress off your rear derailleur hanger(if you cross chain) Generally speaking like everyone says you will be fine with a short cage. I run a med cage on my Campy cross bike, I sometimes end up in my 48 27 combo when I am in the red a few laps in. I have a long cage on my Campy triple set up. I also have run a short cage on my triple as with no shifting problems though. Thats not to say the derailleur wasnt twisted in some combos.