The main difference between these two items is:
18846 has two sets of cutting blades - one for mono & braid (top cutters) and one for wire (lower set by handles). This way you do not damage the line cutters by cutting wire with them! Item 18837 has only one mono & braid cutting blade.
In addition, the fulcrum area on the 18846 is a little sturdier than the 18837.