Context is everything, IMO.
In engineering work, numbers should always be digits. In prose, numbers should be spelled out.
Breakfast at the Thompson's was a busy affair; 12 eggs and 6 rounds of toast for their 3 sets of boistrous twins.
Compared to
Breakfast at the Thompson's was a busy affair; twelve eggs and six rounds of toast for their three sets of boistrous twins.
To me it's pretty clear which of those reads better and more naturally as prose; digits really 'jump out' on the page, and while that is great for engineering texts, it is incongruent and distracting for prose.