Based on what you said you would not be comfortable on a B17 with that bike set up that way, the B17 is for a more upright riding position, your bike is not set up that way. The B15, or the Swallow, was made for your bike set up, as was the Swift; the real difference between the Swallow and the Swift is that the Swallow is made of thinner leather and will need to be tensioned more often and thus will wear out faster; the Swallow is a true old school racing saddle; the Swift will be stiffer due to the thicker leather, take longer to break in, but will last longer.