S26 is more expensive for a reason. As far as I know, Bonna has more sugar than S26. The latter also contains fatty acids like DHA and AHA, specifically concocted to mimic the brain-boosting effect like that in breast milk. But this isn't to say that S26 can be at par with breast milk. Personally, if you want your kid to possibly have more edge, breast milk is still the best, coupled with nurturing activities that can provide holistic development.
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