Prime has more marbling (fat) than choice. Straight grass fed beef is hard to get marbling without any grain. Choice works better on a grill or such.
Most people confuse adding some grain to the diet with grain fed beef. Grain fed is usually from a feed lot (but not always) where there is no grass for them to graze, and get fed grain.
It takes a cows 4 stomachs up to 60 days to turn from one to the other (grass to grain or vice versa), and they usually lose weight during the turn.
I do agree that anything from the smaller suppliers does have a better taste. Only issue is, with ground beef, unless you add fat, the burgers are usually dry.
we are setting up to start our own processing. I have access to a large walk in cooler to age, but do know people who have aged in their own freezers. I figure if I mess up on a cut, I can just turn it into ground beef.