"The profiles of omega-3s, saturated fat, cholesterol, calories, vitamin E and vitamin A are all better in grass-fed products because of what grass contains. Grass contains a whole host of macro and micro-nutrients. When an animal eats those components, they’re subsequently present in their foods. It's not possible for grain-fed animals to get all those components. The other big thing is ruminants - cows and sheep - have digestive systems that are set up for foraging, and not for eating grains. When they eat grain they get sick more easily and require antibiotics. The diet can be supplemented, but it's well known that you don't absorb nutrients as well when they're synthetic versus when you get them through food. The American Cancer Society advocates whole foods because of this.
From an interview with Dr. Steve Marks