Aside from the difference in PUFAs I think that a major component of buying grass-fed is that the animals are much healthier being fed grass, eliminating the need for antibiotics, and a true pasture will have less chemicals and pesticides than grain feeding (unless the grain was organic but that never happens). I really wouldn't worry about smallert animals like chicken who would not bioaccumulate as much, but a cow is a huge animal who will eat a lot of grain over its life and bioaccumulate many more toxins. This is the same reason we eat salmon and sardines and shy away from tuna.
Stabbing conventional wisdom in its face.
Anyone who wants to talk nutrition should PM me!