It's all about configuration. If you go with the 15 rise, you'll be slightly(and I do mean slightly) more upright. The 5 will be a little more forward of a riding position. Think of how it changes where you need to reach in order to hold on your grips. The shorter the stem, the closer you'll be to the bar. The longer the stem, the further. The higher the rise, the more upright you'll be and the opposite for the lower rise(or none)