Brazil and Argentina have a very curious and competitive relationship. However, most brazilians will agree that argentinian steak is superior.

1. Argentinian cuts are better, period. They do a better job separating the parts of the meat that require different preparation methods and that offer more flavor. One perfect example is picanha, which was unknown in Brazil 30 years ago, and now is the standard churrasco cut. It's a part of the rear quarter that has no visible division from the rest of the piece, it isn't a separate muscle, and the butcher has to know where to cut it, mostly by experience.
2. Argentine steak has more (and better distributed) fat. It's way tender and has a much richer and better flavor. The fat is evenly distributed in the meat (it's what we call marmorizada, but I forgot how it's called in english). When it cooks slowly, as it does in a traditional parrilla, the fat melts and helps to cook the meat more evenly, while keeping it moist. When you cut the piece the flavor of the steak's fat comes to your nose, is overwhelming.
3. The cattle is also of a different variety from the one raised here in Brazil. Because we are a hotter and more sunny country, our cattle tends to be taller and thinner, such as the Zebu or Nelore varieties. It's similar (and related to) the cattle that is raised in India. Argentina's cattle is related to the lines raised in Europe and some regions in North America where the weather is colder. The cattle is stocky and has more fat, which helps to keep it warm in the winter. That also makes a world of difference.
4. The pampas are a flat region, and it also helps to keep the meat more tender. Cattle from mountainous regions tends to have stronger muscular tissues, which can be truly delicious, but needs to be cooked in a entirely different way.
5. As a side note, here in Brazil we also have our own techniques for making barbecue that are similar to the ones used in Argentina, and churrasqueiras (our version of the parrillas) are standard feature in many homes in the inner regions of Brazil (mostly anywhere that is not the city of São Paulo or in the coastal cities). There are variations in technique depending on the kind of churrasqueira and the region of the country, but many people already know how to prepare a good steak. Using argentinian or uruguayan steaks just makes it better.