You don't get better by running away from a field of runners.
Actually both are good ways to improve, but most runners don't take advantage of the first, and thereby are weak when it comes to racing top fields.
Bob Kennedy and Todd Williams in their prime didn't sit and kick in US champs, they ran hard and ran away from the fields. Kennedy many years later still has two faster times than Rupp at the 5k. Syndey Maree ran from the front and was the first American under 3:30 for 15, still faster than Centro.