To be fair, breast feeding is said to be best because people assume that since its natural it is perfect, nothing is perfect. It does provide immunity though which is important in preventing sickness and infant deaths whereas formula does not have this. However it can be argued that formula is just as nutritious, if not more, as breastfeeding relies on the mothers health and diet. There are all these studies finding that breast feeding protects against such a thing but there are also studies suggesting there is no difference and that formula is better in the long term.
I don't think it matters in the long term both feeding methods have their advantages and disadvantages but breast is best for immunity which is important.
As for brain development etc experience and learning would play the most important role I think. Also factors like how healthy the mother is while pregnant