The length of the leg should match your running pace. i.e. 10min/mi- go for the long 10" shorts. 9min/mi, 9" shorts. 8min/mi- 8" shorts. etc. The faster you run- the more appropriate the shorter leg length.
Really makes sense to me...new runners feel insecure showing so much more skin, so they go for longer apparel (I did at first as well...stuck to capris even though it was damn hot!). Now, the more serious you are, you tend to be less concerned with what non-runners think of your apparel, so you are more willing to flaunt your lack of modesty since it suits your sport and go with the shorter shorts that simply make running more comfortable.