In context|rhetoric|lang=en terms the difference between antiphrasis and antithesis

is that antiphrasis is (rhetoric) use of a word or phrase in a sense not in accord with its literal meaning, especially for ironic or humorous effect while antithesis is (rhetoric) a device by which two contrasting ideas are juxtaposed in parallel form.

As nouns the difference between antiphrasis and antithesis

is that antiphrasis is (rhetoric) use of a word or phrase in a sense not in accord with its literal meaning, especially for ironic or humorous effect while antithesis is a proposition that is the diametric opposite of some other proposition.