Are "Says who?" and "Who says?" interchangable?

Question

According to free dictionary, "Says who?" is used to express disagreement with or defiance against something that someone has just said. And give this example:

A: "There's no way a pipsqueak like you can win this race!"

B: "Oh yeah? Says who?"

I'm wondering if I can replace it with "Who says?". Will the meaning be changed? Is there any difference between these two phrases?

Update with the definition "Who says?" from the dictionary

used to disagree with a statement or an opinion

A: "You can't just take money from the register like that!"

B: "Who says? I own the business, so it's technically my money."

Answer 1

"Who says?" is a completely normal, grammatical sentence, and can be used in that way as well as other ways.

"Says who?" is an idiom, with non-standard grammar:

First, the inversion is not normal. "Says" often shows inversion in narrative, where the quoted utterance precedes (eg "'There he is,' said Jane"), but it is not normal when nothing precedes it.

Secondly, when it is used non-interrogative form (often as a reply eg. "Says my father!") it is usually invariable and doesn't take normal grammatical forms: "Says you!", "Says me!" (rather than "Say you!" "Say I!").

I would say that it is more emphatic, or even agressive, than "Who says?"

Answer 2

"Says who?" is basically the same thing as: "Who says?"

The first is slightly nicer sounding, and the second can be considered a shortening of "Who says [I can't]? Or some other modal verb.

Both are emphatic, spoken forms and can be used aggressively.

Also, the first is used more by adults and the second is slightly more (whiny) childish.