The SS series of motors make more power than their EX counterparts. The EX 13.5 can create 165W of power, while the SS13.5 can create 195W of power. Additionally, the front end-bell in the SS Series has a larger bearing for increased reliability.
On a plus, Novak has made all the parts of these motors interchangeable. You can take an EX13.5 motor, swap out the bonded rotor for a sintered rotor, and swap the end-bell to one with a large bearing, and you now have a SS13.5 motor.