If you want my answer then Darwin would have chosen both.
A prebiotic is a specialized plant fiber that beneficially nourishes the good bacteria already in the large bowel or colon. In short, it is bacteria food.
A probiotic is actual live bacteria cultures often found in yogurt. The problem with these (and the reasons why certain pills & products have them in the billions) is that a large portion of them are killed in the acid of the stomach when ingested.
Both are super beneficial to the human body, which is why Darwin would have used both if they were readily available to him.
However, me being a lover of Darwinism, I say that, if Darwin had to choose just one, then he would go with a prebiotic. It is just the more beneficial of the two. But most importantly, it is the one most easily obtainable through certain raw foods. Something like asparagus for example. This was always the reason why vegetables are always urged to be a part of every single person's diet.