I buy grass fed beef from specific ranchers, I also buy grain finished beef from specific ranchers or brands, I do ths because there are differences. In tems of cooking, I have found that grass fed and finisehd is leaner and will typically cook slower, but, will dry out sooner and at lower temperatures than grain finished beef. I prefer Northern California cattle for grass finished as it is finished on sweet grass. For grain finished, I get beef from Central CA, as the grain is sweeter, rendering sweeter fat.
I have cooked several grass fed briskets and chucks with excellent results. They can be cooked in very similar ways with regular commodity beef with similar results.