Evoking is generally used to get something done in the material world.
An example would be evoking Sallos to physical appearance and tasking him with making someone love you.
Invoking is generally used to make inner changes to the self.
An example would be invoking Sallos to help you to feel more attractive and confident.
These are the traditional definitions within ceremonial magick. Gods, goddesses, and angels were always invoked, and demons were always evoked because it was believed you needed protection from them, even though you were asking them for things.
However, the religious practice of demonolatry, uses both words a bit differently. According to S. Connolly, demons should only be invoked, and her definition of the word seems to imply that it is a sort of open invitation for the demon to appear, rather than a taking of its traits into yourself. She sees evocation as a “summoning by force,” a kind of gun to the head type of thing, so she calls upon demons in the same way gods and goddesses have been traditionally called upon, that is by invocation.