Meat is almost perfectly substitutable. So going out of your way to ensure that the beef you eat has low carbon impact isn't really doing much good. All you're doing is ensuring that somebody else won't eat the beef you bought and will eat something with higher carbon impact instead.

Where it does matter is when you choose not to eat meat. Less demand for beef means less beef raised, but which cattle are they going to stop raising -- the low carbon or the high carbon ones? My guess is that the high carbon beef is the marginal beef. When you stop eating beef you cause the price to drop, so the ranchers with the highest costs are going to be the first to go out of business. And carbon is one of the main costs of ranching, so I would predict that stopping eating beef would have a greater than average impact on carbon emissions.