DCS334B is just an upgraded version of DCS331B and hence has a more diverse feature set as compared to the latter. With a more powerful brushless engine as compared to the brush motor engine of DCS331B which provides a greater speed of 3200 RPM also provides a smoother transition to that speed.  It also ensures a greater user convenience as compared to DCS331B as it has a built-in LED allowing the user to work in low light environments and also is lighter which allows the user to carry the device around the Jobsite.