The second approach is more scientific. The first approach is adhoc and actually assumes the result of hypotheses. While the first approach may seem to require lower investment initially, since you are trying to prove significance, you are gradually increasing sample size and hence may end up spending more due to iterative approach.
In case of the second one, we are adopting an unbiased approach to problem solving. The second approach defines clearly the sample size needed to reach a conclusion with certain confidence level, and may seem costlier, but in the end will save a lot of money and provide unbiased outcome which can be relied upon