HORL 2 Rolling Knife Sharpener: Super Easy Sharp Knives! 

I did 15 degrees, as recommended by the manufacturer. You really can do either though; you can re-shape a 20 degree edge to a 15 degree one (or vice versa), would take much more time the first time around. But I would stick to what the manufacturer says, and in my case, the Zwillings (and Wusthofs) that I have are all 15 degrees.