Nutrition Journal | Full text | A review of fatty acid profiles and antioxidant content in grass-fed and grain-fed beef
Not only that, but grass-fed beef also has higher levels of Omega 3. Omega 3 is essential for the health of your endocannabinoid system- which is what gets you high on pot (and it does 1000 other things, like killing cancer and fighting depression).
Id also like to think the grass fed cows are able to graze a little bit, which is a plus for my conscience. Edit: Although I don't eat much beef usually.
Any animal is better off and better for you when they eat what they have naturally evolved to eat. Just for the hell of argument, take humans for example. If there was an animal that mostly ate humans, which would you think is better for them? An obese person that eats fast food and candy, or a lean healthy person eating paleo/primal/raw foods? The healthier the animal that you're eating is, the healthier it will be for you.