I think I've talked myself into the rubber one but my reasoning is this - when the ball is in your mouth and your teeth rest against the material - I think the rubber will be more forgiving. And, the wearer can 'bear down' onto the rubber? The breathable one will induce drooling too (and he's not into that).
Yarp, kinda goes rubber if you want muffling - reduced noise, biting down, etc. - and plastic if you want drooling and/or the wearer has issues with restricted breathing and doesn't want to feel 'blocked'.
Thanks afraidnotscared, KG and BB - for now I'm using rope as a gag but I think I might ave to buy both! My indecisiveness ruins me (and my bank balance)