The shorter, Rapid Aero bar allows you to use the entire bar length for mounting accessories, since the tower is inserted below the bar, rather than having a cowl or hood wrapping around the bar.
Be sure to choose the recommended bar length set forth in the fit guide. Longer bars will not fit since the channel underneath the bar, where the tower attaches, does not extend deep enough. If you need a longer loadbar, choose the 400xt application with regular steel square load bars.
Start by reading the instructions for assembling the towers. Before attaching the brackets to the tower, measure the correct distance between the towers to ensure a precise and strong fit. The difference between an amateur and a professional installation is having even bar lengths outboard of the tower.