The 3961 has a solid terminal where the 5422 has the removable nut kind. Looks like the solid terminal is larger in diameter.
Plugs with a solid terminal nut can only be used with the larger, snap-on connector type. The terminal is part of the plugs design and cannot be removed. Most automobiles utilize this type of connection system.
In cases of high vibration, a removable terminal nut may become loose and create an insecure electrical connection. Therefore if a terminal nut is used on marine, ATV, or similar applications, the selection of a plug featuring a solid terminal is preferable, if available.