It seems to me 「馬鹿を言え」 and 「馬鹿を言うな」 are pretty much interchangeable. Corresponding expressions in politer form also exist, 「馬鹿を言いなさい」 「馬鹿を言いなさんな」 So what nuances of meaning do they each have? And what's the difference, if any?
Why do they have similar, if not completely overlapping, meanings when one tells the listener to stop and the other says the opposite? 馬鹿を言うな seems pretty straightforward as a command telling the other party to stop with the nonsense, but what about 馬鹿を言え? This phrase appears nothing but counterintuitive. When someone says 馬鹿を言え, what kind of response do they expect from the listener?