With the unethical ideas aside, it is usually best to err on the side of high impact in initial investigations of a phenomenon. That way, you can ascertain if there is anything going on that is worthy of study. In later investigations you will likely become more interested in the boundary conditions. At that time, you may err on the side of low impact. Keep in mind, however, that these are heuristics.
More people are better than fewer people.
Remember that Sir Ronald's logic rested on the possibility of individual differences canceling one another out. The more people in the study, the more confident you can be that this has taken place. Notice that increasing the sample size will increase the SSw. This is because when there are more participants providing scores, then there are naturally more deviation scores. However, as we can see in Equation 1, boosting the n will decrease the MSw.