"What's more" is definitely more common, at least in the U.S. Contractions are less formal than writing the words out in their entirety. I think English spoken in the U.S. is overall, less formal than the English spoken in the U.K. (for example).
14 oktober 2019
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Heb je je antwoorden nog steeds niet gevonden?
Schrijf je vragen op en laat de moedertaalsprekers je helpen!