Dog with Two Bones

Dog has this bone. It's the most important thing in the world to him; he carries it everywhere he goes. One day he goes down to the water and he sees another dog with a bone reflected in the surface. It's identical to him and his bone, and he wants that bone too. So he opens his mouth to grab it, and his bone falls into the water and disappears. It's gone. And he's left standing there, looking at himself, and he has nothing. Nothing at all.

But...

If the dog could learn, if the dog knew he could have only one bone, and he could do it all over again, which bone would he want?