I have been an active butcher dealing with small, sustainable farms for the last 5 years. The differences between grass and grain finished animals have always intrigued me. I couldn't necessarily say that one is better than the other, but the flavor differences and content of marbling have led me to choose one or the other based on the texture and flavor of the finished product. Consistently, grain finished beef has better marbling. That is because fats are much more concentrated in grains, so grain finishing allows farmers control over their product. Most of the farms I work with use regionally native grains to cut back on the cost from feed lots (most of which use genetically modified crops). Grass fed farmers have to go to greater lengths to ensure fat content in their meat. Having worked with over a dozen grass fed beef farms in my region, very few are able to produce great marbling in their finished cuts. The ones that do use pasture rotation and controlled varieties of grass that will go to seed at different points throughout the year. The grass seed, like grain, is the best method to control the marbling of their cows. I find that grass fed beef from most farms will be better in the fall than any other season because that is when most varieties of grass go to seed here in Tennessee. As far as flavor is concerned, the differences are bountiful. Grain finished beef tends to be balanced but rich. This makes it very suitable for dry aging, as the flavored continue to develop intensity throughout the aging process. At 40 days of dry age, grain finished beef is hard to beat. I also prefer grain fed beef for briskets and braising pieces, I find that the consistent fat content leads to a moister entree and the mild flavor of the beef allows other flavor to shine through. The flavor of good grass fed beef is deceptively rich and gamey. Often, even the raw meat has a deeper purplish color to it. Year round, I prefer grass fed beef for leaner cuts, such as rounds and sirloins. But in the fall, grass fed strips are incredible as well. With sufficient marbling, grass fed beef seems to age well, but the flavor can be overwhelming after 20 days of age. Higher levels of iodine in the grass are the cause of the gamey flavor of grass fed beef, just as acorns are to thank for the delicious flavor of iberico pork. From the perspective of a butcher and chef, grain fed beef is the best way to control your product's fat content for both farmers and chefs, but grass fed beef offers another dimension of flavor that speaks for itself. I have other opinions as to the consequences of mass produced beef that is grain fed throughout it's life, which is why I have talked about grain finished beef instead.
