The Medical Research Council are advising parents that although breastfed babies cry more than those on formula milk, breast is still the best. Surveys have shown that within months of breastfeeding, 2/3 of women stop. Mothers are interpreting their babies cry as an indicator that their milk is not satisfying them, however MRC assures that there is no need for alarm; the babies are just trying to communicate.
It could be that bottle fed babies are overfed making them appear to have better temperaments or to do with the social norms of feeding babies that affects the trends seen. Scientists try to encourage women to breastfeed however essentially it is the individual's choice to decide on their feeding methods.