I would use breast or formula on cereal if you can. It's just better milk. I used formula wherever I could if it was just for DS but if it was a meal for the family I used cows milk. I think it depends on your circumstances too, like Katherine said as the milk amount became more then perhaps it's easier to use cows milk rather than express breast milk, but for me it was easy to grab a carton of formula from the fridge and use that.