My understanding, from a former co-worker who was married to an Argentinian and had visited there many times, is that Argentinian beef is entirely grass fed. Most U.S.-raised beef is corn fed. Even those that start life eating grass will be fed corn for several months before slaughter. The feed has an impact on the taste of the meat. Americans are used to the corn-fed flavor, which is why even grass-fed beef is given corn toward the end.
Some people prefer grass-fed to corn-fed, and vice versa, but it's probably based more on what you're used to than which really tastes better. It's like kids with mac & cheese. They're used to Kraft and that's what they expect. Give them a fancy version most adults would love and they'll turn their noses up at it because it's not what they expect. Eventually they'll come to appreciate the fancy version as better, but it will take a while. I've learned the hard way that kids don't like "fancy".