[hunsaker814]

Why would a person purchase long versus short derailleur? Does this relate solely to the size of wheel that is used on the bike? Please educate me...

 

[sanjuro]

More chain pickup with a long cage.

Can't run a triple chainring setup with a short cage.

 

[TWeerts]

generally:

short cage = 1 chainring up front
med cage = 2 chainrings
long cage = 3 chainrings

this may change due to application.