Originally Posted by dsbrantjr

If you are on the fence whether to get a long- or short-cage derailleur, go for the long cage. While there could theoretically be some degradation in shifting if a longer-than-needed cage is used, as a practical matter it is negligible. If you decide to change your gearing down the road, say to a cassette with a larger big cog, the long cage will give you the option without requiring replacement.