Get the shorter of the two unless you play with the guitar around your knees. I've found the shorter one at www.vanhalenstore.com. I'm not sure who else has the shorter one in stocck.
Alternately, if you were comfortable with the idea, you could buy the longer one and punch/cut a hole in one end to shorten it up. I wound up being happier with another brand of strap, so I just gave up on mine and sold it.