A few months back Kevin Rose interviewed well-known alternative health professional Dr. Andrew Weil. You can check out the video here (skip to about 19min 20s).

As earlier posters noted, dark chocolate has loads of antioxidants. While it is high in fat, the fat that it does contain is a 'good' fat that has protective effects on on the heart and circulation.

The cocoa bean has the highest concentration of antioxidants of any fruit. Antioxidants have been linked to lowering cancer rates and cardiovascular health. Dark chocolate is made with a higher proportion of the cocoa bean than milk chocolate. More cocoa = more antioxidants in your body!

There are 3 reasons why dark chocolate is considered healthier than milk chocolate
1. Cocoa is very high in anti-oxidants. Dark chocolate has more cocoa than milk chocolate.
2. Less fillers. To get the soft sweet creamy taste of milk chocolate uses a lot of sugar, milk and cocoa butter to cut the strong taste of cocoa.
3. The stronger flavor of dark chocolate results in people eating smaller portions.

Milk chocolate has more saturated fat and more sugar.

I have been answering food questions after discovering Mahalo today. One thing I must note with food is that not all food is equal. Not all granny smith apples for instance have the same nutrients, sugars, vitamins, etc. With this being stated first I must say that not ALL dark chocolates are healthier than ALL milk chocolates.

There are two parts to almost all chocolate. The cacao powder and cacao butter. Take the cacao beans from inside a football size and shaped fruit. They are then dried. Now the more heat used to dry them the less nutrient quality they will contain. I get mine from Bali, which are dried by the sun over a period of 2-3 days. They are then fermented for another 1-5 days depending on desired flavor. Then in a room heated to about 105 degrees F. they are mushed into a paste. The room is heated to liquefy the cacao butter as it is mashed which is the fat of the cacao bean. This mush is then placed in press cloths and pressed by a machine which squeezes out 30-97% of the oil depending on how dark the desired end product is. The oil is allowed to cool and becomes as hard a rock but can easily be liquefied at around 93 degrees. The mush which has been squeezed of its oil is dried and becomes a powder. So now we have two separate products.
Cacao butter is the fat and is the main ingredient of white chocolate which in all reality is the most healthy part of the cacao plant altogether. Though almost all white chocolate is heavily dosed with chemical emulsifiers, sugars, and artificial flavorings. Add cacao powder back into melted cacao butter with sweetener maybe nuts or peanut butter and you have chocolate. The less cacao butter the more dark the chocolate. Add dairy product which has been way more than pasteurized and prob comes from unhealthy commercially farmed animals full of antibiotics, steroids, medications, and hormones and you have milk chocolate. Though milk chocolates can be made from organic, grass fed dairy, not contain sugar, synthetic emulsifiers, and be healthier than most dark chocolates. Antioxidants aside....