The ball on the bottom is called a "Tuff Ball" and it is indeed tough. A little harder, and little rougher than a "regular" tennis ball, like the one on top.
Darby has a decided preference for the Tuff Ball. If I throw the regular ball, she will run after it, but more often than not, leave it lying there while she runs back to me. If I throw the tuff ball and regular ball at the same time, she will sniff both of them and pick up the Tuff ball and bring it back.
I wonder what she doesn't like about the regular ball--the feel? Or maybe the smell? Maybe it smells like the veterinarian's office where we got it on Saturday when Darby also got a kennel cough shot--maybe it brings back a bad memory?