Well, no one is going to care about you as much as you. So you can never RELY on the vendor to tell you the truth, or even give due diligence regarding safety. Most of the time they will stay inside of the law of the pertinent regulatory authorities and not much more. They're running a business and most of the time they either don't care, or they don't know that there is anything to care about, relying on what the government or scientists tell them, all of whom have their own agendas separate from public health. My point being, you certainly have the right to ask questions, and you should, ask a ton of them, politely but exhaustively, don't stop until you're satisfied. Then do your own research too and verify the claims they make. My butchers tell me grainfed is perfectly fine, but I know better because I understand the way these animals work and what that grain feeding does to the nutrient profile of the beef. By and large, industry is concerned with moving as much product as possible with the best profit margins possible while staying inside of the law. Very rare is the business that cares as much about quality as they do about their bottom line profits.
So ask them lots of questions, politely, and ignore their attitudes by persisting, be bold and realize that these people are kind of stupid and ignorant, not probably their fault, again, they probabaly don't know that there is something to 'know', and also do your own research and come ask questions here like you're doing if something is confusing. It takes years to change your paradigm and understand what truly makes nutritious food because we've been stupified for so long by our dumbing institutions and socialization.
Yes, I'd go with sprouts, because I personally know uruguay produces kickass beef, and it is certified organic, though the WF grassfed is PROBABLY passable, but unless you contact the farms they source from you don't really know. Enjoy the carne!