Vitamin D2 versus Vitamin D3

A friend asks:
I'm puzzled about vitamin D. I have been taking D3, as widely recommended, but now I hear I should be taking vitamin D2. What's the story?
As far as science knows at this time, our body treats D2 and D3 exactly the same. The only difference is that D3 comes from animal sources, such as sheep fat, and D2 comes from plant sources, such as mushrooms or algae. (Note, only a very specific and specially raised mushroom contains D3.)
With many vitamins, the number at the end can signify an entirely different purpose. Vitamin K1, for example, coagulates blood. Vitamin K2 directs where calcium is stored in the body (making the difference between hard arteries or healthy bones). So K1 and K2 are entirely different!
With vitamin D, the actual function in the body is the same as far as science can tell right now, it is just that D2 is better for the environment and doesn't contribute to animal cruelty.