Medium Cage Vs, Short Cage

So basically my rear derailleur is pretty mangled, so sadly i need to buy another one!
Im debating whether i should go with the ultegra 6700 GS or SS, however im not to knowledgeable with this so can anyone have any suggestions!

Thanks

6700 SS -> for double chainring configuration
http://bike.shimano.com/publish/content/global_cycle/en/us/index/products/road/Ultegra_Grey/product.-code-RD-6700-G-SS.-type-.rd_road.html

6700 GS -> for triple chainring configuration
http://bike.shimano.com/publish/content/global_cycle/en/us/index/products/road/Ultegra_Grey/product.-code-RD-6700-G-GS.-type-.rd_road.html

You only really need the long cage if you are running a cassette with a large capacity or you have a big range in your cranketset (ie a triple or some non-standard chainring setup).

Short cage.

People only go long cage to compensate.