The first is non-standard, mainly American, conversational, casual. The second is normal standard English.
"Of insertion" - that is, using 'of' in this way, e.g. too big of a problem, how hard of a job will it be? is mostly confined to American casual speech, and is hardly ever encountered in written English except in reported speech.
...dialectal “of a” usages are
becoming acceptable idioms in casual speech and informal writing.
However, we still wouldn’t recommend them in formal English, written
In fact, this dialectal construction—like “how long of a drive”—isn’t
found much in print anyway, except in the most casual writing.
Fowler’s Modern English Usage (rev. 3rd ed.) calls it an informal oral
usage that’s confined, so far, to American English.
Merriam-Webster’s says the same: “Our evidence shows the idiom to be
almost entirely oral; it is rare in print except in reported speech.”
As M-W concludes: “The only stricture on it suggested by our evidence
is that it is a spoken idiom: you will not want to use it much in
writing except of the personal kind.”
Not that big of a deal?