basically the lower number gives a less power,but more feel,and more travel.the RCS ones are good as give you options as to what you want from the lever,a lot people run 19/20 ratios because they think it gives more stopping power,which it does to a degree,but at the expense of general feel,they are basically on/off,which makes it hard to trail the brakes.i much prefer the 17/18 on my bike,even though the pistons are larger on my calipers,it gives a lot more control on the brakes.