Translation:It is very cold outside, I will fetch more blankets.
Both would be equally correct. "Det er koldt" is a subjective feeling relating to the person, whereas "Der er koldt" is more objective. A person could say; "Jeg fryser. Det er koldt" relating to the person him/herselvf feeling the cold, but would say "Der er koldt i Sibirien" based on the knowledge that it is a cold place in general.