Both are intended to help users model contact forces in Simscape Multibody models. If the Spatial Contact Force meets your needs, I highly recommend you use that block.
The Simscape Multibody Contact Forces Library was created long before the Spatial Contact Force block was added to Simscape Multibody. We created that library to examine use cases and prioritize features for the Spatial Contact Force block. We are maintaining that library on the File Exchange because it is an example of how you could create your own custom contact force law and custom friction law. If you are stuck in a release prior to when the feature you need was added to the Spatial Contact Force block, you could use that library.
Users of the Simscape Multibody Contact Forces Library should be able to (relatively) easy upgrade their models to the Spatial Contact Force block. There are a few contact types that are not (yet) direcly supported by the Spatial Contact Force block (inner surfaces of cylinders, cones, and spheres) but there are new options being added reach release so keep checking.