For years I knew nothing but Coto, Jumbo and alike, a complete utter disaster.
I buy "ojo de befe" "beef angosto" "beef de chorizo". "ojo de befe" being the most marbled.
I wouldn't buy any of those cuts! Chorizo... hmmmmm maybe. But yeah, I don't like any of those. The best and easiest cut to cook that always turns out tender (unless you are doing something insanely wrong) is entrana (skirt steak). Cook WITH the silverside on and then remove before eating. For tenderizing beef, just coarse salt -- best overnight in the fridge. You could do coarse salt + herbs if you prefer. The other cut we use often is vacio (cook slowly, low heat if you are using your oven) and I like asado (tira de asado). Always cook your meat in the brolier portion of your oven, but the whole cooking it for 2 seconds for a rare cut does not produce a tender steak here. Argentine beef is not aged the same way it is in the States, you're trying to cook a different product the same way you would an American steak. I've gotten used to my beef "a punto" rather than under, and it does have better flavour -- my husband likes to cook his until dry, I still find that horrible.