Before I answer this I would like to explore where chocolate comes from. Much of the chocolate consumed today is in the form of sweetened chocolate that contains cocoa solids, cocoa butter or other fat, and sugar. Milk chocolate is sweetened chocolate that additionally contains milk powder or condensed milk. One of the distinguishing ingredients of chocolate is Cacao. In fact, you can measure the purity of chocolate by its content of Cacao. Cacao has been found to have many medicinal properties.
Chocolate contains phenyl ethylamine, which is a mild mood enhancer. Chocolate can also stimulate the production of serotonin which can balance moods and counter depression. Even the smell of chocolate can stimulate brain waves that are the hallmark of states of relaxation. Chocolate is also known to increase antioxidant levels in the blood (polyphenols and catechins) and the bioflavinoides that are in chocolate have been shown to fortify the elasticity of blood vessels. Milk Chocolate has virtually no antioxidant activity due to milk binding the antioxidants. To take advantage of the antioxidant activity of chocolate you need to have at least 70% cocoa solids and no milk. So for health, the darker the better.
Dark chocolate has been found to reduce the risk of four major diseases: stroke, heart failure, cancer and diabetes. The Cacao bean contains no sugar and between 12 percent and 50 percent fat depending on variety and growth conditions. Cacao beans are around 40 percent fat content (low compared to other nuts). Despite the negative press, there is no evidence to implicate cacao bean consumption with obesity. I would focus more on what is combined with the Cacao to be the culprit. And if you are concerned about allergies. I do many allergy tests. Having an allergy to chocolate is quite rare. It is more typical that a person is allergic to an additional ingredient such as milk, dairy products and/or chemicals mixed into the production of heat-treated processed chocolate bars.
All in all I would say this: chocolate is not bad for you (unless you have a negative reaction to it). And as far as differentiating the benefits of Dark vs. Milk Chocolate, by both science and personal bias, I will have to lean towards dark chocolate. Remember, more isn’t always better and dosage is everything. I do not recommend chocolate every day, but there are many days that a square or two of good quality chocolate can make the day a little brighter.