Hardness of Magnetball ?
The MCRS® Magnetball is based on rewarding your dog for the right behaviour (positive-reinforcement-system http://en.wikipedia.org/wiki/Reinforcement )
That's why a reward must be a real reward for your dog, which means when your dog does not like a certain ball (because of to hard, to slippery, wrong shape, etc.) it is no actual reward. Result is that the (reward) system does not work properly.
Too many trainers stick to their 'old fashion' hard rubber balls where just a few of the dogs realy like to bite in. It is our experience that a normal dog likes a toy who is flexible (allmost no dog likes a hard wooden dumbbell, as reaction they start to chew on it), that's why the MCRS® Magnetball is made from the right hardness. There are 2 versions available: the MBall and the MBall-Light. The MBall is suitable for all real hard biting dogs, the MBall-Light is suitable for puppies, smaller breeds and dogs with lower drives.
So please check out your dog's drives and possibilities honestly, this to make the righ choice of ball.
PS. As a sidenote: your dog will not start to chew on the sleeve because using a flexible ball, the contrare ! A grip is born in a dog, you cannot 'make' a grip, you can just try to improve a grip (with lot's of limitations) !