I have no idea. However, I can tell that Jersey milk, and probably Guernsey milk also, is higher in not only butterfat but in solids non fat too. In other words, there is a lot of nutrition packed into it.
Same butterfat. The two breeds come from the same part of the world and were probably bred for the same purpose. Jerseys come from the island of Jersey and Guernseys come from the island of Guernsey. Both islands are in the British isles and are not far from each other.
Not much. Personally, I prefer the Jersey because it is richer than the others, but realistically you probably wouldn't notice much of a difference between them.
There have been any number of studies on goat and cow milk that show that goat milk is more digestible than cow milk. Many people, babies in particular, that cannot tolerate cow milk do very well on goat milk. These studies are old, but they are still valid. Cows vary a lot in how rich their...
I happen to like Jerseys. A Brown Swiss is a nice cow too. They are a little big but they are a calm gentle cow. One of the best milk cows I have ever had was a Brown Swiss Jersey cross. Another good cow to consider, if you can find one, is a Milking Shorthorn.