I very rarely feed Gracie beef liver. I find it unpleasant to handle and cook (it doesn't smell very nice when its cooking). It's also loaded with Vitamin A, which can be too much of a good thing if fed too often. So I only buy it for Gracie a few times a year, just for variety. She eats chicken liver more often, whenever I buy a whole, fresh chicken.
When I do make beef liver for Gracie I prefer calf liver to regular beef liver. My guess is that because calves have (unfortunately) not lived as long as older cattle, they've accumulated less toxic substances in their organs. On the rare occasion I buy calf liver I get it from my grocery store's frozen food section. It probably would be more nutritious purchased fresh.