Originally Posted by gyozadude

Typically the Shimano SS short arm RDs will handle between 26 - 28t max rear cog size, and pick up around 26t differential in chain ring (max-min) plus rear cog (max - min). The GS versions can handle upto 34t max cog size and pickup more than 32 - 44t differential depending on model/yr. So if you ride a 52-39 front crank (13t differential) and have an 11-23 cassette (12t), you can use the SS. If you have a triple up front and a 11-34 megarange cassette, then GS is the way to go. Some sites, especially Shimano's, will provide specs, or if you shop your LBS, just ask and they should be able to find the specs in their catalogs.