I'm confused about when to use Specularity map (like the one made in CrazyBump) vs Fresnel node. Both are recommended to make materials look more realistic, I use them to combine Diffuse node with Glossy one (white colour), but everytime I'm using Specularity map, material looks too glossy and bright, while Fresnel gives more realistic look. What's the actual difference between that two?
In an ideal situation you would use both actually. The Fresnel node will simulate the real physical property of many surfaces that causes more light reflection grazing angles. Think looking through a glass pane perpendicularly (transparent, see through) or sideways (you will see more reflection).
The specular map will deal with differences of reflectivity along the surface of the material due to uneven or varying surface properties (more polished areas versus more rough surfaces)
To achieve to most realistic results your could mix both with a Color Mix node and tweat blending values to the desired result
Edit: Here are two examples, either add the texture to the roughness input of a glossy shader
Or add the image texture to the mix value of a shader mix node.
Or both, and if desired you can additionally use it for bump mapping if necessary
Actually we cant compare the 2 of them.
Fresnel node is used to make more reflection at high angles,and specular maps are used to define where those reflections should be no matter what the angle is.
Fresnel is mostly used in dilectric materials like PVC plastic along with specular map in some cases.
The right way to make a material is to use both of them if needed.
Making Scratched PVC plastic:
I have a specular map and i want to use it,easy !
Just add the fresnel to the specular map.
And this will give us:
Note: If the specularity is very high when using specular map,just darken it using the rgb curve or other color node.