The answer may require more than one word.
The truth is that we often see both variations in professional, produced scripts .. even ones that aren't written by the director.
A quick search gives:
COP: "Unit 217 requesting back-up"
OWENS: ".. chrono shows we're 22 weeks out, so gravity wasn't supposed to kick in for another 19 ..."
BURBAGE: "Here is two sovereigns"
BERNARD: "Write this number down. It's a 24-hour hotline ..."
SULTAN: "Can go from zero to 100 kilometers an hour in 12.5 seconds. And I even like the color"
DISPATCHER: "Attention all units -- 211 at Pier 39. Suspect is a white male, 30's, 6 feet with specs, armed and very dangerous"
EVE: "She's only 12. Some day she'll understand .."
TITO: "This is it, Jon. For the next 72 hours you are on your own."
ARCHER: "Met Caster Troy in Berkeley at a Pearl Jam reunion. Felony convictions: two counts, drug running. She has a tattoo of a 1956 Buick on her right ankle"
ARCHER: "Dietrich Hassler. 45. Biochemist. Dismissed from the FDA on charges of ..."
etc ..
Mac
It's true that screenwriters do it both ways. I've seen it both ways. I've done it both ways. But generally, and I think it's true in a majority of the cases above, when you use a number in your dialogue, it's a number that can only be read one way.
"12 years old" can only be read as "twelve years old."
1945, meaning a year, can only be read as "nineteen forty-five."
And while a handful of them might be spoken in a couple different ways -- do you say "100" as "one hundred" or "a hundred" -- "211" (which is a police code) as "two one one" or "two eleven" -- for practical purposes, it doesn't really matter all that much.
The over-arching rule is always going to be clarity. What's easiest to read, easiest on the eye?
BOB
The city's population? It's 12,650.
LARRY.
Make it 12,649.
BANG!
That's just bound to be a lot easier on the eye than:
BOB
The city's population? It's twelve thousand, six hundred and fifty.
LARRY
Make it twelve thousand, six hundred and forty-nine.
BANG!
NMS
"12,650" is always going to be easier to read than "twelve thousand, six hundred and fifty" -- even if it isn't exactly according to Hoyle.
NMS