As mentioned, a specific feed does not determine marbling. Marbling is a bodily process involving growth, aging/maturity. When more nutrients are going into the body than what the body needs for growth, it will first start putting on a layer of OUTSIDE fat, then it will put fat around the organs, THEN it finally puts fat (marbling) within the meat. Specific breeds puts on fat deposit in different ways.
There is NO research that proves grass fed is better health wise than grain fed. Actually, all the research has proven grain fed is healthier. No one hears about that because it is not the BUZZ in the media.
Texas A&M has done the only research I am aware of comparing grass vs grain.