I went with the bigger size. For me, i dont intend to have it on the bike except for long trips so i wanted the most storage available. A big advantage also is you can store a full coverage helmet in it so if rain or bug issues occur, you can prepare for it. I also went aftermarket for this item since i only use it once or twice a year. The only quality issue i had was the attaching lock levers were not HD quality and made it a little harder to snap onto the docking system, not a big deal really.