How do you quantify this happiness? You need some objective measure if science is going to provide an answer. Say you adopt a utilitarian approach - Do you ask people 'rate your happiness on a scale of 0-10 for options A and B, and the add up the scores? That's pretty subjective - how do you know my '8 happiness' is the same as your '8 happiness'? Also, the statistics would rely on their being some linear relationship between the values - iow, '8 happiness' needs to be four times as much as '2 happiness'. This seems like a difficult assumption to defend.

Reason can be useful in pointing out the grey areas. For example, if someone is in constant pain with terminal cancer, applying a universal 'thou shalt not kill' moral code seems questionable (to be fair, the code is really 'thou shalt not kill humans', since no-one would think twice about putting an animal out of their suffering). In that case, one could argue that the universal code makes no sense, since the person's quality of life is going to be awful, and no life is almost certainly better than a life of constant suffering with no hope of improvement.

But pointing out the grey areas is about as far as reason can go. It can't quantify things like happiness or well-being or justice, as these are subjective qualities.