I am trying to find out what is better for the kind of riding I do. I do mainly trails. Not to much down hill. I have a hard tail with front shox so trying to find out what would be better a long or short rear derailleur and what is the difference for.
The difference is in how much chain you need to wrap, which is determined by: The sum of the differences between the highest and lowest number of teeth on the biggest and smallest chainwheels, front and rear. For example, a crankset with 22t small ring and 44t large ring paired with an 11-34 cassette requires (44-22)+(34-11)=45t wrapping capability. This number should then be matched or exceeded by the number listed on the derailer. In laymens' terms, if you use a triple crankset get a long cage derailer.
For MTB's Shimano don't do short cage RD's, only medium (GS) and long (SGS) SRAM give the option of all 3 lengths. As Frankenmike you need to know how many T your cassette & crank are, and buy according to that, if you look at the Shimano Tech Docs, you can see what they spec the RD's for.
For MTB's Shimano don't do short cage RD's, only medium (GS) and long (SGS) SRAM give the option of all 3 lengths.
Calling your bluff. I seem to remember there being a shimano saint SS cage deraillieur, and I've had 8 speed xtr short cages in the past. Unless you are riding a DJ/4S/4X/DH bike, it's better to run the proper deraillieur to your gearing.