I would suggest the names SlotWrapperType and MethodWrapperType because I think they match their string representations the closest.

For checking whether something is a method/function one could also use inspect.isroutine (or inspect.ismethoddescriptor), but that is inconsistent with regards to builtin and python methods:

>>> import inspect
>>> inspect.isroutine(object.__init__)
True
>>> inspect.isroutine(object().__init__)
False
>>> class A:
...   def f(self):
...     pass
...
>>> inspect.isroutine(A.f)
True
>>> inspect.isroutine(A().f)
True

Maybe a function to detect the second case is needed.