The obvious pros and cons is that the thinner rubber stores less energy, but is also easier to pull. It probably gives a smoother and more precise shot, although with less penetration power and less absolute range. At what point it becomes overpowered (or underpowered ofc) depends on a lot of factors including the spear lenght/diameter/material, water temperature, gun manufacturer, barrel material, etc. So instead I'd recommend:
What comes standard with the gun? Stick with that.
By the time you wear the bands out you have enough experience to judge whether you want to change them for something else. But really, any gun, in any configuration can get you fish.