Excluding major differences such as soy milk vs cows milk based formulas, or casein vs whey, as I understand it they are all basically the same. Infant formulas need to meet a basic standard, and anything that is proven to be essential to a baby's development must be added to them all. One may taste better than another (to the baby) but unless one is upsetting the baby, IMHO there is no reason to stick to one formula. In my own experience my DS1 was formula fed from 6 momths and I used whatever was on special for him as long as it was the same type, but nor brand, of formula.