I agree with Stephen.
If you are talking normal, run of the mill cereals and breads, then hmmm probably the bread, but I would go with neither.
However, there are a million different points on the scale with some of kelloggs chocolate covered cereals right down to yeast free, gluten free museli or oat based cereals and then highly processed cheap white bread at one end and organic sprouted bread at the other.
If you are going for breads, I would either go for a sprouted bread or a dark rye - and for cereals, something that is gluten and yeast free.
Oh, but then it depends on what you put on your bread or cereal...
Hmmmm, sometimes life is too complicated!!!
Ross