Rule(s) of thumb:
Less than 12" of vertical travel = Short arm
12" or more vertical travel = Long arm
4-4.5" of lift or less = Short arm
More than 4.5" of lift = Long arm
Those would be the two most basic traits to compare the two against, IMO. There are other benefits to long arms, depending on manufacturer (or builder) such as tailored/adjusted suspension geomentry. But at it's simplest form the above to points would be how I would suggest you decide.
2007 2dr X, 6sp, 35's on D44's, 2" lift, cage, hydro, etc.