He 4 is usually used for He cooling, which goes down to roughly 4 K. With He 3 you can get a little bit lower, and He 3 has some properties which makes it more

He 4 is usually used for He cooling, which goes down to roughly 4 K. With He 3 you can get a little bit lower, and He 3 has some properties which makes it more useful at even lower temperatures, but the usual cooling is done with He 4, which is not exactly cheap. (I was once told that liquid Nitrogen is about the price of cola and liquid Helium the price of Whiskey, at least in a very rough sense.)