We've gotten both. As crazy as this may sound, Kaji prefers the ones that are the size of a normal tennis ball.
you might want to try them both and see which one your pup likes better. Mine really doesn't care, as long as they squeak, bounce, and he can chase them. But, I have noticed the small last longer than the xs. He can break the xs the same day he gets them, and the small ones will last him a few weeks. These toys are not hard to find either. You can get them at any Petsmart, or Target. I like the Target ones better, because they come in different colors.
