You need long cage if you have a triple up front. Otherwise short cage should be fine. This would also be a chance to change to an MTB rear derailer if you ever think you might want to run a rear cog bigger than 28. I put a new $22 Shimano Deore (it was on clearance and I forget the exact model number) on my commuter a couple of years ago (I run 7-speed SIS). I normally run 12-21 but change to 11-34 for loaded touring.