Humanoid rig uses more advanced Mecanim features than the Generic rig. You can find the list of such features below:

• Automatic retargeting: This is one of the most important differences between those two rigs. The Humanoid rig uses automatic retargeting, which means that you can have the same animations on different humanoid characters. Your characters can share a group of animations (basic movement or some common actions). It gives you the possibility to buy your animation assets from the Asset Store and easily use them on your characters. Generic rigs don't support this feature. You have to prepare animations for your specific rig or retarget them in a 3D package.
• Inverse kinematics: This feature lets you control your characters' feet and hands position with scripting. It is useful for making your characters stand on uneven ground or grab an object in the scene. It is a built-in feature for Humanoid rigs in Unity. Generic rigs have to use custom-made solutions.
• Advanced animation settings: Humanoid rigs have more settings for animation import, such as the mirror option. We will discuss them in depth in the Looping mirroring and offsetting animations recipe.
• Look at: Unity has a built-in solution for humanoid characters looking at something. You have to write custom systems for generic characters.
• Additional bones: A lot of people think that they cannot use additional bones with Humanoid rigs. It is not true. If your rig has an animated weapon slot for instance, you can still use it with the Humanoid rig. All you need to do is to find the Mask section in your animation import settings and enable the additional bone for each animation it is used in (you need to use the Transform foldout to find your additional bone).

I highly recommend using Humanoid rigs for all humanoid characters in your game.