Not that big a deal / not that big of a deal

Which is correct?
People who think mobile security is ______________ could be in for a rude awakening.

a. not that big a deal
b. not that big of a deal

Answer: a.

Explanation:
“Not that big of a deal” is non-standard grammar. However, it’s increasingly common, especially in the U.S.

This seems like a good one to Google for a snapshot of use. Let’s see …

Not that big a deal: 9,610,000
Not that big of a deal: 21,100,000

Wow. Over twice as many instances of the wrong phrase … and you know what that means: “Not that big of a deal” is going to become standard. Language changes and rules change to follow. There’s nothing we can do to change that, so there’s no point in making too big a deal of it.

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