What is stronger a bb gun or paintball gun?

Neither. Both perform as intended.

Pellet is more accurate and more expensive.

ADDING: If you like to shoot rapid-fire, there are more options for semi-automatic guns among BB guns. A round steel BB is easy to load and to make a gun self-loading (loads into the chamber). Most pellet guns have to be loaded by hand, one shot at a time, although there are a few models that use 5 shot clips that look clumsy to use.