It's a lot more efficent. Well, air cooled V-twin vs. air cooled parallel twin that is. See, there's a bit of a design flaw with an air cooled V-twin, the back cylinder doesn't get the same cooling that the front cylinder gets so both cylinders have to be babied down so the back cylinder doesn't get over heated. That's not the case with a parallel twin, since both cylinders get the same cooling you can maximize performance from both cylinders creating a more efficent motor. The cooling factor goes right out the window if the engines are liquid cooled but since neither the sportster or the America / speedmaster are liquid cooled that's neither here nor there.
