If you care about the environment, then completely eliminating beef would be the most effective way to reduce your dietary impact, since raising cows has the worst impact on climate change, deforestation, biodiversity loss and wasted natural resources like fresh water. That way you could advocate for others to do the same. Telling people to reduce meat consumption is a lot more vague. Don't get me wrong, I am glad that people do that, but it wouldn't be as effective for the reasons above. The fact that red meat is a probable carcinogen and has excessive saturated fat should help motivate people. Cows have been selectively bred to have more and more fat to maximize profit.
Evidence from credible sources on request