I originally bought the medium sized launcher, but I felt the balls were a bit too small for my German Shepherds and worried they'd accidentally swallow one while running with it, so I upgraded to the large.  Not only are the large balls the perfect size for my dogs (although they are quite a bit more expensive than the medium size!), the large launcher also works with the medium sized Cuz dog toys, which is awesome because those are not only difficult to throw, they get really gross and slimy because of the material.