Aah nice one thanks for putting me straight. I was brought up with Black Widow catapults and the like and when I was at school they were always classed as the ultimate catty. I did have a catty with bands once but it was a cheap set up made by one of my mates. The bands just snapped one day and I drilled holes in the tips and pushed tubes through and secured with a ball bearing. I gave it to someone who has been using it ever since for his fishing.
logically your rubber of choice depends wholey on your needs. If you want to hunt with it, then it's best to deliver the most accurate power you can afford/achieve. If it's beer cans and stuff then a long life and minimal power for accuracy is best.
I had a plethora of fun as a boy busting street lights and bean cans with a fishing catapult made for flinding maggots in the drink! It was only when I got older when ' power bands' as they were often advertised came into the scene.
With that in mind, buy the bands that will suit your needs best.