The problem with grass feed beef and bison is that it tastes... well - like grass. The grass would have to be highly controlled to insure low weed and alien grass consumption.
One advantage to grass feed is the Omega 3 fatty acids over Omega 6. O3 is better for skin, joint, cardio and mental health. HOWEVER, recent studies show taking Omega 3 as a supplement has no cardio health benefits. It seems to get the benefit you need to eat it in wild fish and grain feed animals. GO figure
The problem with Buffalo and with Bambi is that has very little fat and small taste.
I far prefer the taste of grass fed over grain fed any day. In fact the burger Ii buy in my town is all local grown grass fed and I gladly pay the extra cash per pound for the good stuff!