Axel, if your passenger(s) are not Amazons (large/tall women) the short sissy bar will work just fine and, again, in my opinion, looks better than the tall one, particularly when paired with Triumph's luggage rack as it is on my '07 Standard, pictured here before Bob Carpenter got his hands on her:
BTW, these are some of the things Mr. Carpenter could do for you, i.e., turn your Rocket III into a truly intergalactic Rocket:
Carpenter Racing
Mine puts about 200 H.P. and 170 ft lbs. of torque to the rear wheel.